diff --git a/Modules/Setup.stdlib.in b/Modules/Setup.stdlib.in
index b12290d436cbeb..fe1b9f8f5380c1 100644
--- a/Modules/Setup.stdlib.in
+++ b/Modules/Setup.stdlib.in
@@ -169,7 +169,7 @@
 @MODULE__XXTESTFUZZ_TRUE@_xxtestfuzz _xxtestfuzz/_xxtestfuzz.c _xxtestfuzz/fuzzer.c
 @MODULE__TESTBUFFER_TRUE@_testbuffer _testbuffer.c
 @MODULE__TESTINTERNALCAPI_TRUE@_testinternalcapi _testinternalcapi.c
-@MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c _testcapi/vectorcall_limited.c _testcapi/heaptype.c _testcapi/unicode.c _testcapi/getargs.c _testcapi/pytime.c _testcapi/datetime.c _testcapi/docstring.c _testcapi/mem.c _testcapi/watchers.c _testcapi/long.c _testcapi/float.c _testcapi/structmember.c _testcapi/exceptions.c _testcapi/code.c
+@MODULE__TESTCAPI_TRUE@_testcapi _testcapimodule.c _testcapi/vectorcall.c _testcapi/vectorcall_limited.c _testcapi/heaptype.c _testcapi/unicode.c _testcapi/getargs.c _testcapi/pytime.c _testcapi/datetime.c _testcapi/docstring.c _testcapi/mem.c _testcapi/watchers.c _testcapi/long.c _testcapi/float.c _testcapi/structmember.c _testcapi/exceptions.c _testcapi/code.c _testcapi/pyos.c
 @MODULE__TESTCLINIC_TRUE@_testclinic _testclinic.c
 
 # Some testing modules MUST be built as shared libraries.
diff --git a/Modules/_testcapi/parts.h b/Modules/_testcapi/parts.h
index c8f31dc8e39fae..60ec81dad2ba9e 100644
--- a/Modules/_testcapi/parts.h
+++ b/Modules/_testcapi/parts.h
@@ -38,6 +38,7 @@ int _PyTestCapi_Init_Float(PyObject *module);
 int _PyTestCapi_Init_Structmember(PyObject *module);
 int _PyTestCapi_Init_Exceptions(PyObject *module);
 int _PyTestCapi_Init_Code(PyObject *module);
+int _PyTestCapi_Init_PyOS(PyObject *module);
 
 #ifdef LIMITED_API_AVAILABLE
 int _PyTestCapi_Init_VectorcallLimited(PyObject *module);
diff --git a/Modules/_testcapi/pyos.c b/Modules/_testcapi/pyos.c
new file mode 100644
index 00000000000000..63140e914875db
--- /dev/null
+++ b/Modules/_testcapi/pyos.c
@@ -0,0 +1,60 @@
+#include "parts.h"
+
+
+static PyObject *
+test_PyOS_mystrnicmp(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    assert(PyOS_mystrnicmp("", "", 0) == 0);
+    assert(PyOS_mystrnicmp("", "", 1) == 0);
+
+    assert(PyOS_mystrnicmp("insert", "ins", 3) == 0);
+    assert(PyOS_mystrnicmp("ins", "insert", 3) == 0);
+    assert(PyOS_mystrnicmp("insect", "insert", 3) == 0);
+
+    assert(PyOS_mystrnicmp("insert", "insert", 6) == 0);
+    assert(PyOS_mystrnicmp("Insert", "insert", 6) == 0);
+    assert(PyOS_mystrnicmp("INSERT", "insert", 6) == 0);
+    assert(PyOS_mystrnicmp("insert", "insert", 10) == 0);
+
+    assert(PyOS_mystrnicmp("invert", "insert", 6) == ('v' - 's'));
+    assert(PyOS_mystrnicmp("insert", "invert", 6) == ('s' - 'v'));
+    assert(PyOS_mystrnicmp("insert", "ins\0rt", 6) == 'e');
+
+    // GH-21845
+    assert(PyOS_mystrnicmp("insert\0a", "insert\0b", 8) == 0);
+
+    Py_RETURN_NONE;
+}
+
+static PyObject *
+test_PyOS_mystricmp(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    assert(PyOS_mystricmp("", "") == 0);
+    assert(PyOS_mystricmp("insert", "insert") == 0);
+    assert(PyOS_mystricmp("Insert", "insert") == 0);
+    assert(PyOS_mystricmp("INSERT", "insert") == 0);
+    assert(PyOS_mystricmp("insert", "ins") == 'e');
+    assert(PyOS_mystricmp("ins", "insert") == -'e');
+
+    // GH-21845
+    assert(PyOS_mystricmp("insert", "ins\0rt") == 'e');
+    assert(PyOS_mystricmp("invert", "insert") == ('v' - 's'));
+
+    Py_RETURN_NONE;
+}
+
+static PyMethodDef test_methods[] = {
+    {"test_PyOS_mystrnicmp", test_PyOS_mystrnicmp, METH_NOARGS, NULL},
+    {"test_PyOS_mystricmp", test_PyOS_mystricmp, METH_NOARGS, NULL},
+    {NULL},
+};
+
+int
+_PyTestCapi_Init_PyOS(PyObject *mod)
+{
+    if (PyModule_AddFunctions(mod, test_methods) < 0) {
+        return -1;
+    }
+
+    return 0;
+}
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 10e507d6b481de..ecd2c5091bdfc8 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -4086,6 +4086,9 @@ PyInit__testcapi(void)
     if (_PyTestCapi_Init_Code(m) < 0) {
         return NULL;
     }
+    if (_PyTestCapi_Init_PyOS(m) < 0) {
+        return NULL;
+    }
 
 #ifndef LIMITED_API_AVAILABLE
     PyModule_AddObjectRef(m, "LIMITED_API_AVAILABLE", Py_False);
diff --git a/PCbuild/_testcapi.vcxproj b/PCbuild/_testcapi.vcxproj
index 4cc184bfc1ac82..439cd687fda61d 100644
--- a/PCbuild/_testcapi.vcxproj
+++ b/PCbuild/_testcapi.vcxproj
@@ -109,6 +109,7 @@
     <ClCompile Include="..\Modules\_testcapi\structmember.c" />
     <ClCompile Include="..\Modules\_testcapi\exceptions.c" />
     <ClCompile Include="..\Modules\_testcapi\code.c" />
+    <ClCompile Include="..\Modules\_testcapi\pyos.c" />
   </ItemGroup>
   <ItemGroup>
     <ResourceCompile Include="..\PC\python_nt.rc" />
diff --git a/PCbuild/_testcapi.vcxproj.filters b/PCbuild/_testcapi.vcxproj.filters
index fbdaf04ce37cb1..0e42e4982c21ff 100644
--- a/PCbuild/_testcapi.vcxproj.filters
+++ b/PCbuild/_testcapi.vcxproj.filters
@@ -57,6 +57,9 @@
     <ClCompile Include="..\Modules\_testcapi\code.c">
       <Filter>Source Files</Filter>
     </ClCompile>
+    <ClCompile Include="..\Modules\_testcapi\pyos.c">
+      <Filter>Source Files</Filter>
+    </ClCompile>
   </ItemGroup>
   <ItemGroup>
     <ResourceCompile Include="..\PC\python_nt.rc">