Calm down now. So far the manga gave us no reason to think like this.

Yeah, there is the tournament arc, but in my opinion I see that arc as a setup arc for the monster arc. Given us a explanation why Metal Bat and Watchdog isn't part of it and other stuff like that.

Or maybe this Orochi character is just a setup for the joke of Saitama one punch the Big Bad. Only to make it more serious when Garou survives Saitama's attacks later on.