All the mythicals may be obtained via the 2016 event celebrating Pokemon’s 20th anniversary. (Deoxys may be obtained in the Delta Episode, postgame.) Here are the legendaries that may only be obtained by transfer through a previous generation:
- Zapdos
- Articuno
- Moltres
- Mewtwo
- Yveltal
- Xerneas
- Zygarde
- Solgeleo
- Lunala
- Cosmog
- Cosmoem
- Type:Null
- Silvally
- Tapu Koko
- Tapu Bulu
- Tapu Lele
- Tapu Fini
Pretty short list, not including gen 7 legendaries. Also, it’s worth noting that Zapdos, Articuno, Moltres, Xerneas, Yveltal and Mewtwo can be obtained in different OR/AS events, most of them having their hidden ability. (Mewtwo, Xerneas, and Yveltal were shiny). However, these events were pretty obscure and I think it’s safe to put them aside as unobtainable in OR/AS.
RED/BLUE/YELLOW (gen 1)
N/A, all legendaries in gen 1 are unobtainable except through transfer or obscure event
GOLD/SILVER/CRYSTAL (gen 2)
- Raikou (Soar with Ho-Oh or Lugia in your party, and the last 2 digits of the time on your 3DS must be 1-20)
- Entei (Soar with Ho-Oh or Lugia in your party, and the last 2 digits of the time on your 3DS must be 21-40)
- Suicune (Soar with Ho-Oh or Lugia in your party, and the last 2 digits the time on your 3DS must be 41-00)
NOTE: They are found east of Rustleburrow and are in a location called “The Trackless Forrest”
- Ho-Oh/Lugia (Ho-Oh is found in Omega Ruby, and Lugia is found in Alpha Sapphire. The way to find them is as follows: Go to Sea Mauville and find the Scanner. Then, go to Mr. Stern, who is located in the docks of Slateport City. He will give you the Clear Bell or Tidal Bell depending on your game. Then, go back to Sea Mauville and you will find a wormhole. Enter it, and capture your legendary.)
RUBY/SAPPHIRE/EMERALD (gen 3)
- Latios/Latias (Gifted to you at level 30 in the main storyline)
- Groudon/Kyogre (You battle it in the main storyline at level 40)
- Rayquaza (Found at Sky Pillar in the Delta Episode at level 70)
DIAMOND/PEARL/PLATINUM (gen 4)
- Heatran (Found in the Scorched Slab)
- Cresselia (Found in Cressent Isle. You find it by streetpassing someone that already has Cressent Isle)
- Uxie (Found in the Nameless Cavern with a Pokemon with maximum friendship in your party, from 8:00 PM - 8:59 PM
- Azelf (Found in the Nameless Cavern with a Pokemon with maximum friendship in your party, from 9:00 PM - 3:59 AM)
- Mesprit (Found in the Nameless Cavern with a Pokemon with maximum friendship in your party, from 4:00 AM - 7:59 PM)
- Palkia/Dialga (Found in the Dimensional Rift near Dewford Town with Mesprit, Uxie, and Azlef in your party)
- Giritina (Found in the Dimensional Rift near Dewford Town with Dialga AND Palkia in your party. Trading is required)
BLACK/WHITE (gen 5)
- Cobalion (Found in the Pathless Plain, with a Pokemon with max EVs in your party, on Sunday, Wednesday, or Friday)
- Terrakion (Found in the Pathless Plain, with a Pokemon with max EVs in your party, on Tuesday or Saturday)
- Virizion (Found in the Pathless Plain, with a Pokemon with max EVs in your party, on Thursday or Monday
- Zekrom/Reshiram (Found near Mauville City with a level 100 Pokémon in your party)
- Kyurem (Found in the Gnarled Den, near Mt. Chimney, with both Zekrom and Reshiram in your party)
- Tornadus/Thundurus (Found with a Castform in your party)
- Landorus (Found with both Tornadus and Thundurus in your party)
X/Y (gen 6)
N/A, All legendaries found in X/Y are unobtainable in OR/AS, probably to encourage more people to purchase X/Y.
S/M (gen 7)
N/A, OR/AS preceded S/M, and as such, gen 7 Pokémon are unobtainable in gen 6.
