I finally had some time so here's an updated patch (including O(1) deletions). I've also added a bunch of notes at the top of odictobject.c. Some notable things left to do:
* address two recently failing tests due to r83881 (issue 17900)
* check for any reference cycles (should be fine)
* validate reentrancy (make sure everything is thread-safe around calls into Python)
* make sure subclassing is okay
* compare performance to dict, pure Python OrderedDict
My goal here is to get an effective OrderedDict that we are happy with, while recognizing that there is room for optimization. However, I won't be okay with faultiness, so the implementation must be complete. This has been my mindset throughout.