According to the T & C on the AA site (same as the oneworld site), open-dated tickets are possible, just not online:

From AA T & C:

Aug 1, 2015

Reservations
Your reservations and ticketing can be handled by any oneworld member airline.
Following your first international flight, all subsequent flights can be left open-dated, allowing you to schedule your trip as you go.

All remaining flights may be left open (i.e. without confirmed flight numbers and dates), as long as each flight is reserved prior to departure. Tickets can be booked or issued up to one hour before departure in some countries. Local service fees may apply. (Open-dated tickets are not available for online booking.)