There are a lot of legendaries available in ORAS, however a lot of them will only show up/be available if certain prerequisites are met.
I know Groudon/Kyogre are available through the Story, and
Rayquaza & Deoxys
through the postgame Delta episode, but what about the other available legendaries? What do I need to do to catch them?
Best Answer
Here's the prerequisites for each legendary encounter. If you accidentally defeat them and save, you may need to rebattle the Elite Four for them to reappear.
Having since tested this again, note that any prerequisite Pokemon don't necessarily need to be caught in 6th generation games as was previously stated.
Tornadus/Thundurus/Landorus
Terrakion/Virizion/Cobalion
Mesprit/Uxie/Azelf
Dialga/Palkia/Giratina
Lugia/Ho-oh
Raikou/Entei/Suicune
Zekrom/Reshiram/Kyurem
Heatran
Latios/Latias (Post Game)
Cresselia
Regice/Registeel/Regirock
These Pokemon need to be unlocked before they can be encountered:
Unlocking:
Watch this guide for visual instructions
Once unlocked
Regigigas
References