I hope people remember this isn't about Saitama facing tougher and tougher adversaries in combat; this series has never been about that. It's about his struggle with being the strongest and no one believing in him, despite being the strongest.
We get the whole shounen bit from all the well done side characters, most of which are really entertaining. However, we fully know and completely expect Saitama to get rid of any character in one punch.
Boros is a strange non exception. First of all, Saitama didn't take him seriously. It is quite the accomplishment of a villain in OPM to force Saitama to get a little more serious. Orochi may possibly be much stronger than Boros, but Saitama already knows how it's going to play out because of Boros. He was just tired of the noise and ended it fast.
No one should've expected any more out of this fight.