How and where can I catch Zekrom and Reshiram in Pokemon Alpha Sapphire and Omega Ruby?
When Soaring around, you may discover a small area near Mauville City which has a star above it. This is the Fabled Cave and inside the cave you will find another portal. In it, you will encounter Reshiram in Omega Ruby or Zekrom in Alpha Sapphire.
The source also contains the locations of all the other legendaries.