With flights booked on AA in economy, non AAdvantage oneworld Emeralds and Sapphires cannot select from the 'defined' premium seats using aa.com.

If your Emerald status is correctly entered in the booking, you should be able to contact AA and select better seating (such as exit rows).

Expertflyer/KVS will show those premium seats that are available (which on aa.com show as 'taken').