The interpretation of techniques into different, practical applications. This is more advanced than (but often confused for) Kata Bunkai. Oyo may be learned through the practice of bunkai, or as a follow-on from bunkai.
For example in Taikyoku Shodan, this would be defending and countering an attacker to your left, then grabbing them and throwing them to the ground behind you (the turn), but retaining a hold on their arm (the Gedan Barai Uke), then stomping on them (the step through Oi Tzuki).
Another example is, rather than the Uchi Uke in Tekki Shodan behind a block, the sequence may be interpreted as spinning the opponent, turning them into a choke hold, then using the Uchi Uke motion to apply the choke until the opponent submits or falls.
Oyo is taking the techniques and applying them to learn new, less obvious or hidden techniques.