Now this is a bit late but I just found out about this so I thought why not write about this. Players playing through Pokemon Omega Ruby and Alpha Sapphire will be glad to know that starters from Johto, Onova and Sinnoh are obtainable in Pokemon Omega Ruby and Alpha Sapphire. The Pokemon are not version exclusive but you can only get one starter of each region in one playthrough.
Obtaining them is very simple, follow the steps below:
Step 1. Defeat the Elite 4.
Step 2. Complete the Delta Episode.
Step 3. Defeat the Elite 4 again.
Step 4. Now each time you defeat the Elite 4 after completing the Elite Episode, Professor Birch will call you up for help.
Step 5. You’ll be in Littleroot Town after beating the Delta 4 so go on towards Route 101.
Also read: Pokemon ORAS Tip: Mega Stone Location Guide
Here Professor Birch will be running in circles from his wife and ask for help allowing you to choose a starter. After that he’ll realise he was being chased by his wife and let you keep the Pokemon. You can do this three times to get the Johto, Unova and Sinnoh starters in respective order.