Does Omega Ruby and Alpha Sapphire take place at the same time as Fire Red or Leaf Green?

Omega Ruby and Alpha Sapphire take place in a parallel universe/alternative timeline than the original Ruby and Sapphire.

Zinnia from ORAS once said this;
"And about the existence of another world, which we have long observed to be just like this one and yet not the same...That's right. A Hoenn region that's almost exactly like this one we live in. Filled with Pokémon and people like us. A world where maybe the evolution of Pokémon took a slightly different path, where Mega Evolution is unknown"

The "parallel Hoenn" she's referring to would be the Hoenn region from Ruby and Sapphire, where Mega evolution doesn't exist.

If XY is in the same timeline as the previous games (Which it most likely is), it doesn't contradict the events in ORAS since they took place in different universes.

I don't think so.

i don't know but i think so