One Punch Man has a lot of interesting plot points while often relying a lot on humor, and the connection between Saitama and King is one of the best examples of that. While Saitama is extremely powerful but usually has a hero rank that doesn't fit his abilities, King ended up as an S-Rank hero without any powers and due to sheer luck, which is a running gag in the manga.
Furthermore, a lot of One Punch Man fans doubt whether King stole Saitama's S-Rank, especially considering how some events have played out in the story. In that regard, the question remains in the fandom that King reached that position thanks to his friend's exploits.
Disclaimer: This article contains spoilers for the One Punch Man series. Any opinion expressed here belongs to the author.
King did not steal Saitama's S-Rank in One Punch Man
Despite the comedic element of this question in One Punch Man, the truth of the matter is that King didn't steal Saitama's S-Rank in the series. First of all, King already had a position as a hero long before Saitama decided to join the Hero Association, so it's fair to say that his gimmick as an accidental legend has been going on for quite some time.
No, there have been moments where Saitama has defeated opponents and King has gotten the recognition but this is mostly due to the running gag of the series of the latter always getting victories without any logical explanation.
In that regard, this is something that happens to King throughout the entirety of the One Punch Man series, not just with Saitama, which is worth pointing out.
There is also an argument to be made that King and Saitama have opposite running jokes, with the former getting a ton of success and recognition as a hero while having no powers and the latter getting no attention or promotions despite being the strongest character in the entire series.
It can't be said that King stole Saitama's promotions because the former has been doing this for quite some time without even noticing it and everyone's favorite bald hero has never had an issue with that.
King and Saitama's character arcs in the series
As mentioned earlier, there is a very strong argument that King and Saitama's characters in One Punch Man are meant to be direct opposites and that is shown through their respective running gags.
Saitama gets to defeat anyone with ease and no one takes him seriously while King is afraid and can't fight but everybody believes he is an amazing hero and one of the strongest individuals out there.
Furthermore, another element that connects the two of them is the fight that Saitama can never defeat King in video games and the latter often serves as someone who can give tips and points of view. This is something that a lot of people managed to see in the second season of the anime, with King offering some words of wisdom despite him being a comedy character.
Their dynamic is one of the most interesting in the series and the two of them can also work on their own, which is something that a lot of anime-only fans are probably going to find out in the third season.
Final thoughts
No, King didn't steal Saitama's victories and role as an S-Rank Hero since the former never meant to do that to the latter. It was a set of coincidences, which is something that has been a running gag throughout the entirety of the One Punch Man series.