I will tell you what I tell the students (M3s and M4s) that I advise in the medical school from which I graduated. You absolutely need a good letter from a PM&R doctor to submit with ERAS before you interview...if you want to get any quality interviews at the good programs.
You can get a letter before you even rotate. I had a letter from my mentor in PM&R and pain/spine medicine before I ever did my first externship because I took time during off days on my M3 rotations that were easier to shadow her and start doing research with her. This totally helped me get exposure to the field, and I was not scrambling for letters when it came time. I had letter from 5 Physiatrists (including the head of inpatient rehab unit at my #2 program and the PD director from my #1 program and the program I matched to) when it came time for applications. I used 3 of these letters with each application and 1 letter from my Cardiology attending who I knew loved me and rec me for Honors.
I matched to a top 5 program, and I had interviews from the arguable top 10 programs in the country as ranked publicly and within the field. I know that my letters made a huge difference, and programs knew me on a personal/professional level much better because of my strong letters.
I would not use an OB/GYN letter unless it is just stellar and the doc knows you very well. OB/GYN does not relate as well to PM&R as Neuro, Ortho, IM, Peds, Psych or even Surgery. Peds, IM, Neuro, and Ortho are best bets for outside letter IMHO.
Hope this helps. PM me if you need any further advice. I would be happy to read Personal Statements or give any advice to help good students be more successful matching into our field.