What site to do checkin on?
 

I booked an American Airlines flight via British Airways with avios. I then went to finnair and moved the booking to my AA account. When it comes time to check in, do I do it on AA or BA?

Check-in is always done through the carrier.

On your AA app/website. I assume you have the AA PNR as well now, right?

Check-in is always done through the operating carrier....

Yes, it will be the OPs carrier. 😀

Yes, it will be the OPs operating carrier...

:)

Not the marketing carrier, or the carrier that sold the ticket, or the carrier that they logged in to to change the FF#, but the operating carrier...

Yes, it probably pays to cross the t’s and dot the other thingies. It’s probably good to spell it out.

As long as the OP checks in with the airline that he’s flying with will be good. 😀 (whew)

Booking is on my AA app, and I have control as far as seating, etc.

The app it’s in is irrelevant. As JJeffrey said, what matters is the operating carrier of the first segment. Check in with that airline.

The Operating Carrier takes care of the checkin. I know that some airline websites are smart and forwards you to the operating carrier's checkin portal. If AA is operating the first segment of your flight you would checkin with AA and they may or may not allow you to checkin online or if they do they may not allow you to print out your boarding passes and it will spit out a "Airport Checkin Required See Agent page" as they will need to verify your travel documents!

If AA were operating only the Domestic portion you would be checked in for the Domestic portion but the international portion would need the agent to verify the checkin documents as well.