diff --git a/dlls/opcservices/package.c b/dlls/opcservices/package.c
index c865cf35add..6e1a024c19e 100644
--- a/dlls/opcservices/package.c
+++ b/dlls/opcservices/package.c
@@ -861,9 +861,6 @@ static HRESULT opc_part_create(struct opc_part_set *set, IOpcPartUri *name, cons
 {
     struct opc_part *part;
 
-    if (!name)
-        return E_POINTER;
-
     if (!opc_array_reserve((void **)&set->parts, &set->size, set->count + 1, sizeof(*set->parts)))
         return E_OUTOFMEMORY;
 
@@ -898,6 +895,21 @@ static HRESULT opc_part_create(struct opc_part_set *set, IOpcPartUri *name, cons
     return S_OK;
 }
 
+static struct opc_part *opc_part_set_get_part(const struct opc_part_set *part_set, IOpcPartUri *name)
+{
+    BOOL is_equal;
+    size_t i;
+
+    for (i = 0; i < part_set->count; ++i)
+    {
+        is_equal = FALSE;
+        if (IOpcPartUri_IsEqual(part_set->parts[i]->name, (IUri *)name, &is_equal) == S_OK && is_equal)
+            return part_set->parts[i];
+    }
+
+    return NULL;
+}
+
 static HRESULT WINAPI opc_part_set_QueryInterface(IOpcPartSet *iface, REFIID iid, void **out)
 {
     TRACE("iface %p, iid %s, out %p.\n", iface, debugstr_guid(iid), out);
@@ -944,11 +956,28 @@ static ULONG WINAPI opc_part_set_Release(IOpcPartSet *iface)
     return refcount;
 }
 
-static HRESULT WINAPI opc_part_set_GetPart(IOpcPartSet *iface, IOpcPartUri *name, IOpcPart **part)
+static HRESULT WINAPI opc_part_set_GetPart(IOpcPartSet *iface, IOpcPartUri *name, IOpcPart **out)
 {
-    FIXME("iface %p, name %p, part %p stub!\n", iface, name, part);
+    struct opc_part_set *part_set = impl_from_IOpcPartSet(iface);
+    struct opc_part *part;
 
-    return E_NOTIMPL;
+    TRACE("iface %p, name %p, out %p.\n", iface, name, out);
+
+    if (!out)
+        return E_POINTER;
+
+    *out = NULL;
+
+    if (!name)
+        return E_POINTER;
+
+    if ((part = opc_part_set_get_part(part_set, name)))
+    {
+        *out = &part->IOpcPart_iface;
+        IOpcPart_AddRef(*out);
+    }
+
+    return *out ? S_OK : OPC_E_NO_SUCH_PART;
 }
 
 static HRESULT WINAPI opc_part_set_CreatePart(IOpcPartSet *iface, IOpcPartUri *name, LPCWSTR content_type,
@@ -959,6 +988,17 @@ static HRESULT WINAPI opc_part_set_CreatePart(IOpcPartSet *iface, IOpcPartUri *n
     TRACE("iface %p, name %p, content_type %s, compression_options %#x, part %p.\n", iface, name,
             debugstr_w(content_type), compression_options, part);
 
+    if (!part)
+        return E_POINTER;
+
+    *part = NULL;
+
+    if (!name)
+        return E_POINTER;
+
+    if (opc_part_set_get_part(part_set, name))
+        return OPC_E_DUPLICATE_PART;
+
     return opc_part_create(part_set, name, content_type, compression_options, part);
 }
 
diff --git a/dlls/opcservices/tests/opcservices.c b/dlls/opcservices/tests/opcservices.c
index ab8c2e47a57..2bcf46fb9b7 100644
--- a/dlls/opcservices/tests/opcservices.c
+++ b/dlls/opcservices/tests/opcservices.c
@@ -42,16 +42,17 @@ static void test_package(void)
     static const WCHAR uriW[] = {'/','u','r','i',0};
     static const WCHAR rootW[] = {'/',0};
     IOpcRelationshipSet *relset, *relset2;
+    IOpcPartUri *part_uri, *part_uri2;
     IOpcPartSet *partset, *partset2;
+    OPC_COMPRESSION_OPTIONS options;
     IStream *stream, *stream2;
+    IOpcPart *part, *part2;
     IOpcRelationship *rel;
-    IOpcPartUri *part_uri;
     IOpcFactory *factory;
     IOpcPackage *package;
     LARGE_INTEGER move;
     ULARGE_INTEGER pos;
     IUri *target_uri;
-    IOpcPart *part;
     char buff[16];
     IOpcUri *uri;
     HRESULT hr;
@@ -80,11 +81,55 @@ static void test_package(void)
     hr = IOpcFactory_CreatePartUri(factory, uriW, &part_uri);
     ok(SUCCEEDED(hr), "Failed to create part uri, hr %#x.\n", hr);
 
+    hr = IOpcFactory_CreatePartUri(factory, uriW, &part_uri2);
+    ok(SUCCEEDED(hr), "Failed to create part uri, hr %#x.\n", hr);
+
+    part = (void *)0xdeadbeef;
     hr = IOpcPartSet_CreatePart(partset, NULL, typeW, OPC_COMPRESSION_NONE, &part);
     ok(hr == E_POINTER, "Unexpected hr %#x.\n", hr);
+    ok(part == NULL, "Unexpected pointer %p.\n", part);
 
-    hr = IOpcPartSet_CreatePart(partset, part_uri, typeW, OPC_COMPRESSION_NONE, &part);
+    hr = IOpcPartSet_CreatePart(partset, part_uri, typeW, OPC_COMPRESSION_NONE, NULL);
+    ok(hr == E_POINTER, "Unexpected hr %#x.\n", hr);
+
+    hr = IOpcPartSet_CreatePart(partset, part_uri, typeW, 0xdeadbeef, &part);
     ok(SUCCEEDED(hr), "Failed to create a part, hr %#x.\n", hr);
+    hr = IOpcPart_GetCompressionOptions(part, &options);
+    ok(SUCCEEDED(hr), "Failed to get compression options, hr %#x.\n", hr);
+    ok(options == 0xdeadbeef, "Unexpected compression options %#x.\n", options);
+
+    part2 = (void *)0xdeadbeef;
+    hr = IOpcPartSet_CreatePart(partset, part_uri, typeW, OPC_COMPRESSION_NONE, &part2);
+    ok(hr == OPC_E_DUPLICATE_PART, "Unexpected hr %#x.\n", hr);
+    ok(part2 == NULL, "Unexpected instance %p.\n", part2);
+
+    part2 = (void *)0xdeadbeef;
+    hr = IOpcPartSet_CreatePart(partset, part_uri2, typeW, OPC_COMPRESSION_NONE, &part2);
+    ok(hr == OPC_E_DUPLICATE_PART, "Unexpected hr %#x.\n", hr);
+    ok(part2 == NULL, "Unexpected instance %p.\n", part2);
+    IOpcPartUri_Release(part_uri2);
+
+    hr = IOpcPartSet_GetPart(partset, NULL, NULL);
+    ok(hr == E_POINTER, "Unexpected hr %#x.\n", hr);
+
+    part2 = (void *)0xdeadbeef;
+    hr = IOpcPartSet_GetPart(partset, NULL, &part2);
+    ok(hr == E_POINTER, "Unexpected hr %#x.\n", hr);
+    ok(part2 == NULL, "Unexpected pointer %p.\n", part2);
+
+    hr = IOpcPartSet_GetPart(partset, part_uri, NULL);
+    ok(hr == E_POINTER, "Unexpected hr %#x.\n", hr);
+
+    hr = IOpcPartSet_GetPart(partset, part_uri, &part2);
+    ok(SUCCEEDED(hr), "Failed to get part, hr %#x.\n", hr);
+    IOpcPart_Release(part2);
+
+    hr = IOpcFactory_CreatePartUri(factory, targetW, &part_uri2);
+    ok(SUCCEEDED(hr), "Failed to create part uri, hr %#x.\n", hr);
+
+    hr = IOpcPartSet_GetPart(partset, part_uri2, &part2);
+    ok(hr == OPC_E_NO_SUCH_PART, "Unexpected hr %#x.\n", hr);
+    IOpcPartUri_Release(part_uri2);
 
     hr = IOpcPart_GetContentStream(part, NULL);
     ok(hr == E_POINTER, "Unexpected hr %#x.\n", hr);
diff --git a/include/opcbase.idl b/include/opcbase.idl
index 8523271217c..d8507535634 100644
--- a/include/opcbase.idl
+++ b/include/opcbase.idl
@@ -43,7 +43,9 @@ typedef [v1_enum] enum
 
 cpp_quote("#define OPC_E_NONCONFORMING_URI MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0x1)")
 cpp_quote("#define OPC_E_RELATIONSHIP_URI_REQUIRED MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0x3)")
+cpp_quote("#define OPC_E_DUPLICATE_PART MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0xb)")
 cpp_quote("#define OPC_E_INVALID_RELATIONSHIP_TARGET MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0x12)")
+cpp_quote("#define OPC_E_NO_SUCH_PART MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0x18)")
 cpp_quote("#define OPC_E_NO_SUCH_RELATIONSHIP MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0x48)")
 cpp_quote("#define OPC_E_ENUM_COLLECTION_CHANGED MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0x50)")
 cpp_quote("#define OPC_E_ENUM_INVALID_POSITION MAKE_HRESULT(SEVERITY_ERROR, FACILITY_OPC, 0x53)")