And how will Omastar get a shell smash off against an offensive team? Yeah, choice locked Fire-moves (or Ice), like the V-create on the scarf set you suggested.
In the process of setting up a shell smash, Omastar will be heavily damaged by Dragon Claw, followed by the potential extremespeed. I don't even understand why we're discussing this, as Omastar won't likely pull off a sweep unless we elect to use scarf-mons locked into the wrong moves. And you mentioned earthquake which does indeed OHKO. You won't set-up with a Rayquaza against Omastar, where is the logic in that?
252+ Atk Life Orb Rayquaza Earthquake vs. 4 HP / 0 Def Omastar: 286-338 (101.41 - 119.85%) -- guaranteed OHKO
Dragon Claw calc:
252+ Atk Life Orb Rayquaza Dragon Claw vs. 4 HP / 0 Def Omastar: 172-203 (60.99 - 71.98%) -- guaranteed 2HKO
Also, heavy damage. If Omastar smashed, it will be killed by E-speed.
Reaching a mediocre #56 spot on PS doesn't validate anything either.