From Bulbapedia, the community-driven Pokémon encyclopedia.
The following is a list of areas & items in ORAS that are initially beyond your reach but become accessible later after obtaining the appropriate HMs and/or reaching the appropriate point in the storyline. Note that this does not list areas that are only visited after a needed HM is (or should have been) obtained.
After defeating Norman
After encountering Groudon/Kyogre in the Cave of Origin
After defeating the Elite Four
After completing the Delta Episode
After defeating the Elite Four twice
After seeing every non-event Pokémon in the Hoenn Pokédex
After catching every non-event Pokémon in the National Pokédex