I am in the middle of a DONE5 with several BA segments coming up. Until last week neither me nor my partner could select seats on some BA flights (something about the flights having reached their seat allocation limit), and a lot of seats were blocked on others (upper deck exits for example). After Dec 5, my partner could choose from almost any seat on the seat map while I couldn't choose any (we both have the same status). Turns out my AA frequent flyer in Amadeus was only entered for LA and QF, but not for BA (this shows at the top of the itinerary on checkmytrip.com). Even re-entering the number on BA.com didn't fix it, but a call to BA did.

Now, I can also choose from any seat on the BA seat maps, including those previously blocked.

From my experience, it seems to me like things have improved for oneworld elites.