How I find the BA-equivalent booking reference for an American Airlines-managed reservation?

There are two ways, depending on the kind of BA flight you booked with AA.

If you booked the BA flight under the AA flight number, eg American Airlines flight 6543, Operated by British Airways, then all you need to do is add your British Airways Executive Club number to the booking. After a few minutes, log into your BAEC account, and look at your upcoming bookings. You'll find in there your BA booking reference for the BA legs, and will be able to select your seat, see avios to be earnt etc.

if you booked the BA flight under the BA flight number, eg British Airways flight BA 345, then for some reason your BAEC details won't get passed over. What you need to do is, bizarrely, go to the Qantas "Manage My Booking" page, and enter your American Airlines reference. Scroll down to the Seats section of the page, and you'll see something like:

Seat selection is only available for flights operated by Qantas. To select seats and manage your booking, visit British Airways site with your booking reference #12ABCD

Then take that booking reference shown there, go to the BA website, use that with Manage My Booking, then enter your BAEC details + pick seats + see avios details + etc

In summary:

• BA flight with AA flight number - enter BAEC details, wait, see on BAEC account
• BA flight with BA flight number - give Qantas AA reference, Qantas will tell you the BA reference

i found a way that seems very reliable and is entirely online, for bookings made by AA:

First, have tripcase (owned by Sabre) link to the AA-created itinerary:

1. Go to tripcase.com (you might need to create an account and log in)
2. Click Create New Trip
3. Enter a name for the trip in the pop-up box and click Create
4. Click Link a Booking (on the upper right)
5. Enter your last name and the AA-issued PNR in the form and click Link

Now, for this or any trip:

1. View the trip in Tripcase (click the trip name under "My Trips")
2. Click Documents on the top center
3. Click Arranger Itinerary

The PNRs for the operating carriers should appear at the top, right after the AA PNR.

This is an addition to the thread after a long interval, and so may be anachronistic in multiple ways.

At least as of this moment, entering one's AA Record Locator into the "Booking Reference" field of www.britishairways.com --> Manage Booking --> Find Booking (along with the passenger's last name) returns the BA Record Locator in the "Booking Reference" field. Hope this helps someone.

This is strange that through the BA Manage My Booking tool you cannot access your reservation. According to multiple resources you should be able to access it.

These sources suggest different ways to pick a seat. BA uses Amadeus for its bookings so if this does not work on BA tools you can try Amadeus' CheckMyTrip, the tool made to consult an Amadeus booking to obtain your BA PNR (to be identified on its website). Another possibility is to read your e-ticket and search for a reference to an Amadeus PNR (it has 6 alphanumeric characters). Finally there is a suggestion that BA may let only oneworld Sapphire and Emerald members pick a seat. And apparently BA bookings include AA PNR when you log on BA.com so maybe it works also the other way around, when logging on AA.com you might find the BA PNR.

It's sadly true that it can be quite difficult to obtain the partner PNRs for tickets issued by AA. Here are a few ways:

• In the itinerary email sent by AA on request, the BA locator should appear somewhere within the information for each BA flight, but you may have to hunt for it (see screen shot below)
• You can call AA, give the agent the AA locator, and ask for your BA locator
• You can call BA, give the agent your first BA flight date and number, and the agent can look you up by name on that flight and tell you your BA locator

In addition, as noted in other answers, often the BA site will accept your AA locator, but I didn't include it above because it can't be relied on to always work.

Here's a screen shot of an AA email that contains the BA locator:

I just came to this site and this worked for me:

I checked in to aa.com and re-saved my contact email and phone. This updated the reservation and I then got an email from BA 1 minute later showing the 6-digit BA locator and I was able to add it to my BA iphone app using that locator. Unexpected but it worked.