Metang - #0375 - Pokédex

Nintendo confirms Pokemon not in the Galar Pokedex can't be

Arceus - #493 - Pokédex

Pokémon Omega Ruby & Alpha Sapphire - National Pokédex

Pokédex Locations - #0999 Gimmighoul

Azurill - #298 - Pokédex

Goodra - #706 - Pokédex

Garganacl - #934 - Pokédex

Pokémon X & Pokémon Y - Kalos Dex - Coastal Pokédex

Pokédex, Ultimate Pop Culture Wiki

Here is a map I made with the location and info for all catchable Pokemon in Omega Ruby and Alpha Sapphire : r/pokemon