One Punch Man is a deconstruction manga about the superhero tropes, notably being too powerful. Of course, Saitama is too powerful even in his own verse, but does that means he's a nigh-unstoppable invincible character? **** no. The strongest we've seen One Punch Man (as a verse) reach was on a planetary level via ONE's statements about Saitama. It doesn't mean it's where his limit ends, just that he could do it. Definitely possible that he's stronger, but since we don't know how strong, he's just sitting at an at least type of level that destroys planets, along with Boros, Garou, and probably the newer antagonist God.
The farthest I can see Saitama go is the Saiyan and Namek saga before Frieza casually walks in and says "hello have you heard about our lord and savior uhhhh
me" before whacking him to death.
Even weaker people in power can defeat Saitama by differing powers just because they ignore your own conventional durability. An example I hold to heart with this is Tusk, using its infinite energy to completely obliterate you and your soul from the inside-out. Or at least that's the tl;dr. But in power, the thing can barely bust a building.
Of course, people like Goku aren't invincible either, but saying Saitama destroys all of DBZ or just anyone in general because he was made to be strong is classified as a No-Limits Fallacy, which is like how ScrewAttack said Superman had no limit to power or how Ganondorf can't be killed with anything but the Master Sword. Though they did do the Hulk's theoretical infinite strength growth well as showing how its an NLF, only due to physical limits of the body.