A good question. WOA and WOP are divisions of each other if you will for White Oak. If I understand things right, RRA and WOA use the same barrels (Wilson) and both use the Wylde chamber which means it will shoot 223 or 5.56 well. The main difference is White Oak Arms and White Oak Precision offer pinned rear sights.The nice thing about pinned sights is they don't move around and wiggle like a normal A-2 sight will. White Oak Precision offers a better barrel and the owner, John does the work. WOP also uses a Wylde chamber which I really like.
I have two WOP uppers and could not be happier with them. I bought a RRA NM lower and then when my upper came in, pushed two pins and had a top shelf Match AR. The rifle shot well from the start. I made Master a few matches later. Then this upper has a lot of mileage so I ordered another WOP upper this time with a Kreiger barrel. Hoo boy. I never knew what the rave was about but this barrel makes no copper.
The first upper then got it's own lower, again a RRA NM. I use this rifle as a loaner, Shoots good enough still I can hook you or I use it for reduced matches and save the Kreiger upper for full a across the course matches.