Is it possible to get exit row seating on American Airlines when one checks in online?

Not unless you have status (Gold or above) in AA's FF program.

We've only been able to get it when checking in at the airport.

Thanks--Statia, do u have Gold or above status with AA?

If you don't have elite status you can get exit row seats at airport check in if they are available. They are in high demand though and are usually taken by elite travellers well before check in time. They sometimes become available the day of travel if those elite flyers who were assigned to those seats get upgraded to Business or First Class so it pays to ask at the time you check-in.

You can ask again at the gate if at first you don't succeed at check-in.

Thanks all-- guess I'll give it a try at check-in and then again at the gate.