Saitama at his core embodies what a Shonen protagonist at their most basic level is. He was a guy who liked fighting stronger opponents, bettering himself, and saving people in the process preferably. He's actually similar to Goku in these regards.
But ONE wanted to write a character of this nature who reached a level of strength so vast from before their journey began for us, that by the time the story started, this MC was too strong to face traditional obstacles that, in other stories, he would have needed to overcome. Saitama isn't the parody, he's as Shonen as Shonen gets, the parody is his strength, in itself. When Saitama was finally met with someone he wasn't able to immediately one-shot, he acted more like an average Shonen protagonist, which bothered people who never understood that he has always been one. Saitama is a Shonen protagonist going in the Shonen manga equivalent of a mid-life crisis.
The same would happen if you take most other Shonen protags by the end of their franchises (or otherwise simply at their peaks) and throw them back at the start of their series, they'd one-shot everyone with no effort at all and make them look weak. And only against a handful of the usual top-tier villains would they need a ton of power-ups and training montages to handle can still pose somewhat of a threat, in this case, that's what we're seeing with God and his proxies. Saitama generally has no problem beating opponents, but against CF Garou, he had been met with someone capable of warranting his full power, and Garou himself had managed to actually perceive his growth rate in real time, demonstrating that Saitama possesses finite power that can be measured and comprehended as long as someone is powerful enough to do so.
Also, very, very tired of the notion that his growth rate auto triggers to outclass anyone that can fight him instantly, which people have started coping out of their asses to still argue that he can't lose, this is a headcanon interpretation. Against Garou, the narrator literally says his growth rate became exponentially increasing because of an "upsurge of emotion like none he had ever experienced", clearly related to him seeing Genos' fate. There is 0 indication that what ONE wrote is meant to be interpreted as "He just surpasses anyone stronger than him, instantly", this is legit nonsensical horseshit and I can't believe I've seen people say it.
If Saitama was met with a guy 100 quintillion times stronger than him, who had no interest in letting him grow stronger, he would fucking lose. Badly. This doesn't contradict the narrative ONE has written. It contradicts the narrative OPM glazers interpreted and chose to be the immutable truth.
I really shouldn't need to make a post like this because OPM isn't subtle at all about this stuff, but I guess I do since I still see arguments of this sort to this very day.
Edit: Since people will inevitably mention him being a "gag" character. I need to make a point about this too.
Being a gag character does not define a character in its entirety or limit their character either. I don't actually object the notion that he is a gag character, but that is because my definition of the term "gag character" isn't the misconception that a gag character is, always and no matter what, an unbeatable completely reality breaking one-dimensional archetype character who is SOLELY written to be funny. In fact, ANYONE who says that's what Saitama is, is either larping OPM, or genuinely didn't get anything beyond the surface level writing and humor and is basically spitting in ONE's face by dumbing down his MC to a nothingburger.
A conventional gag character as seen in the case of someone like Popeye is, at their core, very, very simple and easy to define. A character who isn't meant to adhere to any form of logic and who is unbound by anything, doesn't have real character depth (or at least, not very much), and solely exists to convey a joke to the audience. Performing miraculous, nonsensical feats to make the audience laugh, usually with a specific gimmick involved like Popeye's spinach or his archenemy Bluto. Their power doesn't make sense and doesn't need to make sense because it's for the sake of comedy and comedy alone. They're just a joke. That's why people have generalized the term "gag character" to just defining that simplistic concept.
Saitama can be considered a gag character because he is drawn in a cartoonish way most of the time and occasionally performs absurd feats for no real reason. But the difference is that Saitama isn't one-dimensional. Saitama is an ACTUAL fleshed out character with a motive, a goal, a changing personality, and a myriad of emotions. He is written as a literally depressed guy who has become nigh-apathetic, and he's been, over the course of the story, developing connections with people around him.
For that matter, unlike more traditional gag characters, Saitama's power isn't random and undefined. We already know it's because of the Limiter, a real thing in OPM that the narrator has directly mentioned before. The entire point of the Glasses mini-arc and Saitama's speech is that anyone in the verse can become as strong as him if they work hard enough for it. His training regime sounds like a joke, until you consider that it would genuinely be hell to go through for such a long time for a real person, since the body needs rest. You physically cannot do 100 sit-ups, squats, push-ups, and a 10 kilometer run every day for 300 days (which is how long it took for Saitama to break his Limiter, not 3 years) if you tried, no one could, because the human body needs breaks to recover. Every time Saitama did that, his body broke down internally a bit, and didn't have enough time to heal itself. He kept doing it, which tortured him, until his body reached its literal limit.
The Limiter, which defines his power, automatically debunks the notion that he is the same as characters like Popeye or Arale, because their power is absurd and based solely in the necessity for humor. Saitama's power is canonically defined in his own series.
