Fellowships withour residency
Only few specialties accept fellowship without residency. These some weird specialties such as interventional cardiology, transplant, research, ...etc
The best resource to find about medical fellowships in USA is the freida website
Go to this page
https://freida.ama-assn.org/Freida/user/viewProgramSearch.do
and choose obgyn fellowships and then go to each individual program website and read their website or even email them if you see any glance of hope.
This is a long time process and you have to devote at least 6 hours searching for what you want.
But then again as Aviram told you, you chances are very low if not nil.
By the way don't forget that you have to have ECFMG even if you get a fellowship without residency.
Good luck