If you are connecting from CO to OAL, then CO is paying the OAL when the OAL bills CO for the ticket it lifted. That's probably not the case if you're traveling OAL to CO.

At AA, if you can show the AA agent that you arrived within 24 hours of departure (e.g., OAL boarding pass) you are either marked as a through and not charged the GB. If you forget to do that or if the agent makes a mistake when you're checked in, you can send the boarding pass stub to NRSA (billing) to have the GB refunded.