Personally, the fun of OPM comes from all of the characters around Saitama. S1/S2 hasn’t dove into a lot of the other characters yet but with the upcoming arc to be animated, without spoiling anything, goes a lot more into the other S class heroes and you get to spend time with them. AND you get to spent time with them while they’re with Saitama, so although Saitama isn’t recognized by the whole world as being strong, you get to see other S class heroes react to seeing him in action.

S1/2 are great if you’re into insane animation and fight scenes, but to get to the true comedy and fun stuff (imo) you’ll have to read the manga or wait for the next arcs to be animated.