> PyObject_TypeCheck() should be used instead of PyObject_IsInstance() (see
> issue24257).

Thanks for pointing this out.  I've fixed both dictobject.h and odictobject.h.

>
> Perhaps Py_ODict_GetItemId() should be private API as relevant dict function.

This isn't needed for 3.5, right?  I'm not opposed to adding more
functions to the C API for OrderedDict, but I wanted to start out as
small as possible.  My main motivation for the C implementation was
for use in the interpreter and I hadn't given much thought to the
direct use of C OrderedDict by extension modules.  That can be
addressed in 3.6 if it's important.  I supposed you could double-check
with Larry if you want to add more odict support to the C-API for 3.5.