diff --git a/lld/COFF/Config.h b/lld/COFF/Config.h
--- a/lld/COFF/Config.h
+++ b/lld/COFF/Config.h
@@ -40,6 +40,13 @@
 static const auto ARMNT = llvm::COFF::IMAGE_FILE_MACHINE_ARMNT;
 static const auto I386 = llvm::COFF::IMAGE_FILE_MACHINE_I386;
 
+enum class ExportSource {
+  Unset,
+  Directives,
+  Export,
+  ModuleDefinition,
+};
+
 // Represents an /export option.
 struct Export {
   StringRef name;       // N in /export:N or /export:E=N
@@ -58,8 +65,7 @@
   StringRef forwardTo;
   StringChunk *forwardChunk = nullptr;
 
-  // True if this /export option was in .drectves section.
-  bool directives = false;
+  ExportSource source = ExportSource::Unset;
   StringRef symbolName;
   StringRef exportName; // Name in DLL
 
diff --git a/lld/COFF/DLL.cpp b/lld/COFF/DLL.cpp
--- a/lld/COFF/DLL.cpp
+++ b/lld/COFF/DLL.cpp
@@ -555,9 +555,10 @@
 // A chunk for the export descriptor table.
 class ExportDirectoryChunk : public NonSectionChunk {
 public:
-  ExportDirectoryChunk(int i, int j, Chunk *d, Chunk *a, Chunk *n, Chunk *o)
-      : maxOrdinal(i), nameTabSize(j), dllName(d), addressTab(a), nameTab(n),
-        ordinalTab(o) {}
+  ExportDirectoryChunk(int i, int j, int k, Chunk *d, Chunk *a, Chunk *n,
+                       Chunk *o)
+      : baseOrdinal(i), maxOrdinal(j), nameTabSize(k), dllName(d),
+        addressTab(a), nameTab(n), ordinalTab(o) {}
 
   size_t getSize() const override {
     return sizeof(export_directory_table_entry);
@@ -568,14 +569,15 @@
 
     auto *e = (export_directory_table_entry *)(buf);
     e->NameRVA = dllName->getRVA();
-    e->OrdinalBase = 1;
-    e->AddressTableEntries = maxOrdinal;
+    e->OrdinalBase = baseOrdinal;
+    e->AddressTableEntries = (maxOrdinal - baseOrdinal) + 1;
     e->NumberOfNamePointers = nameTabSize;
     e->ExportAddressTableRVA = addressTab->getRVA();
     e->NamePointerRVA = nameTab->getRVA();
     e->OrdinalTableRVA = ordinalTab->getRVA();
   }
 
+  uint16_t baseOrdinal;
   uint16_t maxOrdinal;
   uint16_t nameTabSize;
   Chunk *dllName;
@@ -586,8 +588,10 @@
 
 class AddressTableChunk : public NonSectionChunk {
 public:
-  explicit AddressTableChunk(COFFLinkerContext &ctx, size_t maxOrdinal)
-      : size(maxOrdinal), ctx(ctx) {}
+  explicit AddressTableChunk(COFFLinkerContext &ctx, size_t baseOrdinal,
+                             size_t maxOrdinal)
+      : baseOrdinal(baseOrdinal), size((maxOrdinal - baseOrdinal) + 1),
+        ctx(ctx) {}
   size_t getSize() const override { return size * 4; }
 
   void writeTo(uint8_t *buf) const override {
@@ -595,8 +599,8 @@
 
     for (const Export &e : ctx.config.exports) {
       assert(e.ordinal != 0 && "Export symbol has invalid ordinal");
-      // OrdinalBase is 1, so subtract 1 to get the index.
-      uint8_t *p = buf + (e.ordinal - 1) * 4;
+      // Subtract the OrdinalBase to get the index.
+      uint8_t *p = buf + (e.ordinal - baseOrdinal) * 4;
       uint32_t bit = 0;
       // Pointer to thumb code must have the LSB set, so adjust it.
       if (ctx.config.machine == ARMNT && !e.data)
@@ -612,6 +616,7 @@
   }
 
 private:
+  size_t baseOrdinal;
   size_t size;
   const COFFLinkerContext &ctx;
 };
@@ -634,8 +639,9 @@
 
 class ExportOrdinalChunk : public NonSectionChunk {
 public:
-  explicit ExportOrdinalChunk(const COFFLinkerContext &ctx, size_t i)
-      : size(i), ctx(ctx) {}
+  explicit ExportOrdinalChunk(const COFFLinkerContext &ctx, size_t baseOrdinal,
+                              size_t i)
+      : baseOrdinal(baseOrdinal), size(i), ctx(ctx) {}
   size_t getSize() const override { return size * 2; }
 
   void writeTo(uint8_t *buf) const override {
@@ -643,13 +649,14 @@
       if (e.noname)
         continue;
       assert(e.ordinal != 0 && "Export symbol has invalid ordinal");
-      // This table stores unbiased indices, so subtract 1 (OrdinalBase).
-      write16le(buf, e.ordinal - 1);
+      // This table stores unbiased indices, so subtract OrdinalBase.
+      write16le(buf, e.ordinal - baseOrdinal);
       buf += 2;
     }
   }
 
 private:
+  size_t baseOrdinal;
   size_t size;
   const COFFLinkerContext &ctx;
 };
@@ -831,12 +838,15 @@
 }
 
 EdataContents::EdataContents(COFFLinkerContext &ctx) : ctx(ctx) {
-  uint16_t maxOrdinal = 0;
-  for (Export &e : ctx.config.exports)
-    maxOrdinal = std::max(maxOrdinal, e.ordinal);
+  unsigned baseOrdinal = 1 << 16, maxOrdinal = 0;
+  for (Export &e : ctx.config.exports) {
+    baseOrdinal = std::min(baseOrdinal, (unsigned)e.ordinal);
+    maxOrdinal = std::max(maxOrdinal, (unsigned)e.ordinal);
+  }
+  assert(baseOrdinal >= 1);
 
   auto *dllName = make<StringChunk>(sys::path::filename(ctx.config.outputFile));
-  auto *addressTab = make<AddressTableChunk>(ctx, maxOrdinal);
+  auto *addressTab = make<AddressTableChunk>(ctx, baseOrdinal, maxOrdinal);
   std::vector<Chunk *> names;
   for (Export &e : ctx.config.exports)
     if (!e.noname)
@@ -851,9 +861,10 @@
   }
 
   auto *nameTab = make<NamePointersChunk>(names);
-  auto *ordinalTab = make<ExportOrdinalChunk>(ctx, names.size());
-  auto *dir = make<ExportDirectoryChunk>(maxOrdinal, names.size(), dllName,
-                                         addressTab, nameTab, ordinalTab);
+  auto *ordinalTab = make<ExportOrdinalChunk>(ctx, baseOrdinal, names.size());
+  auto *dir =
+      make<ExportDirectoryChunk>(baseOrdinal, maxOrdinal, names.size(), dllName,
+                                 addressTab, nameTab, ordinalTab);
   chunks.push_back(dir);
   chunks.push_back(dllName);
   chunks.push_back(addressTab);
diff --git a/lld/COFF/Driver.cpp b/lld/COFF/Driver.cpp
--- a/lld/COFF/Driver.cpp
+++ b/lld/COFF/Driver.cpp
@@ -372,7 +372,7 @@
       if (!exp.extName.empty() && !isDecorated(exp.extName))
         exp.extName = saver().save("_" + exp.extName);
     }
-    exp.directives = true;
+    exp.source = ExportSource::Directives;
     ctx.config.exports.push_back(exp);
   }
 
@@ -1045,6 +1045,7 @@
     e2.data = e1.Data;
     e2.isPrivate = e1.Private;
     e2.constant = e1.Constant;
+    e2.source = ExportSource::ModuleDefinition;
     ctx.config.exports.push_back(e2);
   }
 }
@@ -2241,7 +2242,7 @@
       if (!e.forwardTo.empty())
         continue;
       e.sym = addUndefined(e.name);
-      if (!e.directives)
+      if (e.source != ExportSource::Directives)
         e.symbolName = mangleMaybe(e.sym);
     }
 
diff --git a/lld/COFF/DriverUtils.cpp b/lld/COFF/DriverUtils.cpp
--- a/lld/COFF/DriverUtils.cpp
+++ b/lld/COFF/DriverUtils.cpp
@@ -549,6 +549,8 @@
 // Used for parsing /export arguments.
 Export LinkerDriver::parseExport(StringRef arg) {
   Export e;
+  e.source = ExportSource::Export;
+
   StringRef rest;
   std::tie(e.name, rest) = arg.split(",");
   if (e.name.empty())
@@ -641,6 +643,19 @@
   return sym;
 }
 
+static StringRef exportSourceName(ExportSource s) {
+  switch (s) {
+  case ExportSource::Directives:
+    return "source file (directives)";
+  case ExportSource::Export:
+    return "/export";
+  case ExportSource::ModuleDefinition:
+    return "/def";
+  default:
+    llvm_unreachable("unknown ExportSource");
+  }
+}
+
 // Performs error checking on all /export arguments.
 // It also sets ordinals.
 void LinkerDriver::fixupExports() {
@@ -671,19 +686,36 @@
   }
 
   // Uniquefy by name.
-  DenseMap<StringRef, Export *> map(ctx.config.exports.size());
+  DenseMap<StringRef, std::pair<Export *, unsigned>> map(
+      ctx.config.exports.size());
   std::vector<Export> v;
   for (Export &e : ctx.config.exports) {
-    auto pair = map.insert(std::make_pair(e.exportName, &e));
+    auto pair = map.insert(std::make_pair(e.exportName, std::make_pair(&e, 0)));
     bool inserted = pair.second;
     if (inserted) {
+      pair.first->second.second = v.size();
       v.push_back(e);
       continue;
     }
-    Export *existing = pair.first->second;
+    Export *existing = pair.first->second.first;
     if (e == *existing || e.name != existing->name)
       continue;
-    warn("duplicate /export option: " + e.name);
+    // If the existing export comes from .OBJ directives, we are allowed to
+    // overwrite it with /DEF: or /EXPORT without any warning, as MSVC link.exe
+    // does.
+    if (existing->source == ExportSource::Directives) {
+      *existing = e;
+      v[pair.first->second.second] = e;
+      continue;
+    }
+    if (existing->source == e.source) {
+      warn(Twine("duplicate ") + exportSourceName(existing->source) +
+           " option: " + e.name);
+    } else {
+      warn("duplicate export: " + e.name +
+           Twine(" first seen in " + exportSourceName(existing->source) +
+                 Twine(", now in " + exportSourceName(e.source))));
+    }
   }
   ctx.config.exports = std::move(v);
 
diff --git a/lld/test/COFF/Inputs/ordinals-override.def b/lld/test/COFF/Inputs/ordinals-override.def
new file mode 100644
--- /dev/null
+++ b/lld/test/COFF/Inputs/ordinals-override.def
@@ -0,0 +1,3 @@
+EXPORTS
+foo @55
+bar @66
diff --git a/lld/test/COFF/export.test b/lld/test/COFF/export.test
--- a/lld/test/COFF/export.test
+++ b/lld/test/COFF/export.test
@@ -15,7 +15,7 @@
 
 CHECK2:      Export Table:
 CHECK2-NEXT: DLL name: export.test.tmp.dll
-CHECK2-NEXT: Ordinal base: 1
+CHECK2-NEXT: Ordinal base: 5
 CHECK2-NEXT: Ordinal      RVA  Name
 CHECK2-NEXT:       5   0x1008  exportfn1
 CHECK2-NEXT:       6   0x1010  exportfn2
@@ -26,7 +26,7 @@
 
 CHECK3:      Export Table:
 CHECK3-NEXT: DLL name: export.test.tmp.dll
-CHECK3-NEXT: Ordinal base: 1
+CHECK3-NEXT: Ordinal base: 5
 CHECK3-NEXT: Ordinal      RVA  Name
 CHECK3-NEXT:       5   0x1008
 CHECK3-NEXT:       6   0x1010  exportfn2
@@ -52,7 +52,7 @@
 
 CHECK5:      Export Table:
 CHECK5-NEXT: DLL name: export.test.tmp.dll
-CHECK5-NEXT: Ordinal base: 1
+CHECK5-NEXT: Ordinal base: 2
 CHECK5-NEXT: Ordinal      RVA  Name
 CHECK5-NEXT:       2   0x1010  fn2
 CHECK5-NEXT:       3   0x1008  exportfn1
diff --git a/lld/test/COFF/export32.test b/lld/test/COFF/export32.test
--- a/lld/test/COFF/export32.test
+++ b/lld/test/COFF/export32.test
@@ -26,7 +26,7 @@
 
 # CHECK2:      Export Table:
 # CHECK2-NEXT: DLL name: export32.test.tmp.dll
-# CHECK2-NEXT: Ordinal base: 1
+# CHECK2-NEXT: Ordinal base: 5
 # CHECK2-NEXT: Ordinal      RVA  Name
 # CHECK2-NEXT:       5   0x1008  exportfn1
 # CHECK2-NEXT:       6   0x1010  exportfn2
@@ -38,7 +38,7 @@
 
 # CHECK3:      Export Table:
 # CHECK3-NEXT: DLL name: export32.test.tmp.dll
-# CHECK3-NEXT: Ordinal base: 1
+# CHECK3-NEXT: Ordinal base: 5
 # CHECK3-NEXT: Ordinal      RVA  Name
 # CHECK3-NEXT:       5   0x1008
 # CHECK3-NEXT:       6   0x1010  exportfn2
@@ -66,7 +66,7 @@
 
 # CHECK5:      Export Table:
 # CHECK5-NEXT: DLL name: export32.test.tmp.dll
-# CHECK5-NEXT: Ordinal base: 1
+# CHECK5-NEXT: Ordinal base: 2
 # CHECK5-NEXT: Ordinal      RVA  Name
 # CHECK5-NEXT:       2   0x1010  fn2
 # CHECK5-NEXT:       3   0x1008  exportfn1
diff --git a/lld/test/COFF/ordinals-override.test b/lld/test/COFF/ordinals-override.test
new file mode 100644
--- /dev/null
+++ b/lld/test/COFF/ordinals-override.test
@@ -0,0 +1,182 @@
+# RUN: yaml2obj %s -o %t.obj
+#
+# RUN: lld-link /out:%t.dll /dll %t.obj
+# RUN: llvm-objdump -p %t.dll | FileCheck --check-prefix=CHECK1 %s
+#
+# CHECK1:      Export Table:
+# CHECK1:      DLL name: ordinals-override.test.tmp.dll
+# CHECK1:      Ordinal base: 1
+# CHECK1:      Ordinal      RVA  Name
+# CHECK1-NEXT:       1   0x1010  ?bar@@YAXXZ
+# CHECK1-NEXT:       2   0x1000  ?foo@@YAXXZ
+# CHECK1-NEXT:       3   0x1020  baz
+#
+# RUN: lld-link /out:%t.dll /dll %t.obj /EXPORT:?foo@@YAXXZ,@55
+# RUN: llvm-objdump -p %t.dll | FileCheck --check-prefix=CHECK2 %s
+#
+# CHECK2:      Export Table:
+# CHECK2:      DLL name: ordinals-override.test.tmp.dll
+# CHECK2:      Ordinal base: 55
+# CHECK2:      Ordinal      RVA  Name
+# CHECK2-NEXT:      55   0x1000  ?foo@@YAXXZ
+# CHECK2-NEXT:      56   0x1010  ?bar@@YAXXZ
+# CHECK2-NEXT:      57   0x1020  baz
+#
+# RUN: lld-link /out:%t.dll /dll %t.obj /EXPORT:?foo@@YAXXZ,@55 /EXPORT:?bar@@YAXXZ,@122
+# RUN: llvm-objdump -p %t.dll | FileCheck --check-prefix=CHECK3 %s
+#
+# CHECK3:      Export Table:
+# CHECK3:      DLL name: ordinals-override.test.tmp.dll
+# CHECK3:      Ordinal base: 55
+# CHECK3:      Ordinal      RVA  Name
+# CHECK3-NEXT:      55   0x1000  ?foo@@YAXXZ
+# CHECK3-NEXT:     122   0x1010  ?bar@@YAXXZ
+# CHECK3-NEXT:     123   0x1020  baz
+#
+# RUN: echo "EXPORTS" > %t.def
+# RUN: echo "?foo@@YAXXZ @55" >> %t.def
+# RUN: echo "?bar@@YAXXZ @122" >> %t.def
+# RUN: lld-link /out:%t.dll /dll %t.obj /DEF:%t.def 2>&1 | FileCheck --check-prefix=WARN --allow-empty %s
+# WARN-NOT: lld-link: warning
+#
+# RUN: llvm-objdump -p %t.dll | FileCheck --check-prefix=CHECK3 %s
+#
+# RUN: lld-link /out:%t.dll /dll %t.obj /DEF:%t.def /EXPORT:?foo@@YAXXZ,@10000 2>&1 | FileCheck --check-prefix=DUPLICATED %s
+# DUPLICATED: lld-link: warning: duplicate export: ?foo@@YAXXZ first seen in /export, now in /def
+#
+# RUN: llvm-objdump -p %t.dll | FileCheck --check-prefix=CHECK4 %s
+#
+# CHECK4:      Export Table:
+# CHECK4:      DLL name: ordinals-override.test.tmp.dll
+# CHECK4:      Ordinal base: 122
+# CHECK4:      Ordinal      RVA  Name
+# CHECK4-NEXT:     122   0x1010  ?bar@@YAXXZ
+# CHECK4-NEXT:   10000   0x1000  ?foo@@YAXXZ
+# CHECK4-NEXT:   10001   0x1020  baz
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_AMD64
+  Characteristics: [  ]
+sections:
+  - Name:            .drectve
+    Characteristics: [ IMAGE_SCN_LNK_INFO, IMAGE_SCN_LNK_REMOVE ]
+    Alignment:       1
+    SectionData:     2020202F6578706F72743A62617A3D3F62617A4040594158585A202F44454641554C544C49423A224C4942434D5422202F44454641554C544C49423A224F4C444E414D455322202F4558504F52543A3F666F6F4040594158585A202F4558504F52543A3F6261724040594158585A20
+  - Name:            '.debug$S'
+    Characteristics: [ IMAGE_SCN_CNT_INITIALIZED_DATA, IMAGE_SCN_MEM_DISCARDABLE, IMAGE_SCN_MEM_READ ]
+    Alignment:       1
+    SectionData:     04000000F1000000670000002900011100000000433A5C6769745C746573745F6F7264696E616C735C6275696C645C6C69622E6F626A003A003C1101620000D000130021008F7B0000130021008F7B00004D6963726F736F667420285229204F7074696D697A696E6720436F6D70696C65720000
+    Subsections:
+      - !Symbols
+        Records:
+          - Kind:            S_OBJNAME
+            ObjNameSym:
+              Signature:       0
+              ObjectName:      'C:\git\test_ordinals\build\lib.obj'
+          - Kind:            S_COMPILE3
+            Compile3Sym:
+              Flags:           [ NoDbgInfo, SecurityChecks, HotPatch ]
+              Machine:         X64
+              FrontendMajor:   19
+              FrontendMinor:   33
+              FrontendBuild:   31631
+              FrontendQFE:     0
+              BackendMajor:    19
+              BackendMinor:    33
+              BackendBuild:    31631
+              BackendQFE:      0
+              Version:         'Microsoft (R) Optimizing Compiler'
+  - Name:            '.text$mn'
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       16
+    SectionData:     C20000CCCCCCCCCCCCCCCCCCCCCCCCCCC20000CCCCCCCCCCCCCCCCCCCCCCCCCCC20000
+  - Name:            .chks64
+    Characteristics: [ IMAGE_SCN_LNK_INFO, IMAGE_SCN_LNK_REMOVE ]
+    SectionData:     D8FCB3B2D22EA6A98C9353D17E359992135E943A2071A7060000000000000000
+symbols:
+  - Name:            '@comp.id'
+    Value:           17136527
+    SectionNumber:   -1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+  - Name:            '@feat.00'
+    Value:           2147549584
+    SectionNumber:   -1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+  - Name:            '@vol.md'
+    Value:           2
+    SectionNumber:   -1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+  - Name:            .drectve
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          111
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        3793361033
+      Number:          0
+  - Name:            '.debug$S'
+    Value:           0
+    SectionNumber:   2
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          116
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+  - Name:            '.text$mn'
+    Value:           0
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          35
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        1911911868
+      Number:          0
+  - Name:            '?foo@@YAXXZ'
+    Value:           0
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            '?bar@@YAXXZ'
+    Value:           16
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            '?baz@@YAXXZ'
+    Value:           32
+    SectionNumber:   3
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_FUNCTION
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            .chks64
+    Value:           0
+    SectionNumber:   4
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_STATIC
+    SectionDefinition:
+      Length:          32
+      NumberOfRelocations: 0
+      NumberOfLinenumbers: 0
+      CheckSum:        0
+      Number:          0
+...