Static Encounter: Roaring-Sea Caves (Overcast)
Max Raid Battle: Ballimere Lake (Only in Sword), Frigid Sea (Only in Sword), Giant's Bed (Only in Sword) + 2 more
It appears in any weather at Ballimere Lake, Frigid Sea and Giant's Bed + 2 more and only in overcast weather at Roaring-Sea Caves.
Its base catch rate is 45 out of 255 (higher is easier); the catch rate calculator works out your odds with each ball. For the look, the best colour match is Dive Ball or Quick Ball.
Evolution: Omanyte → Omastar (Level 40).
Omastar in every game · Pokémon Sword & Shield interactive map (Galar)