Nothing can ever beat The One Above All. Which probably answers the, “Will he appear in the MCU?” question. There would probably never ever be a reason why The One Above All would need to show himself within the MCU. Unless the entire universe was as the risk of being destroyed, it seems very unlikely.
While the Beyonder and the One-Above-All are both immensely powerful reality-warpers of the highest order, the One-Above-All is clearly the more powerful of the two.
There are numerous incidents within the series that suggest Hulk himself is either the host for the One Below All or may even be the entity itself. This is only half-true, as Hulk is actually the one who manages to defeat the entity, by seemingly killing its host—Brian Banner.
In terms of powers, The One Above All's are limitless, so much so that they are beyond the scope of understanding. If Saitama were to punch him, the blow would not even phase him. In fact, the only way to even weaken One Above All is to cause some form of multiversal imbalance.
In the Jim Starlin's Thanos Infinity Conflict, Thanos has absorbed TOAA along with all multiversal entities becoming the most powerful being in the comics. So yes, Thanos won the battle.
Only he has the power to command the Judge of the Multiverse, the Living Tribunal to which the latter yields. Above all (pun intended), he is supremely powerful as he is infinitely above all of creation in the Marvel Omniverse.
Boros. Boros was the first on-screen opponent of Saitama that survived after getting punched, much to his surprise. In his fight with Saitama, Boros was hit with 3 normal punches, a Consecutive Normal Punches attack, as well as the shockwave from the hero's Serious Punch.
Immense Durability: Boros was shown to be amazingly resilient, to the point of being the first opponent of Saitama to survive after being hit directly by his normal punch, despite having his full power sealed by his armor.
As Wanda climbs the ranks of power in the multiverse, she could surpass the point of power that makes her a literal god, untouchable and decisive, getting her a one way ticket to stand before the One-Above-All.
The one above all is far above zeno, zeno can erase people out of existence but not the one above all because he is out of space and time. Zeno is not omnipotent while the one above all is. The one above all is transparent meaning he cant even be touched by anything so zeno would not be able to even do anything to him.
The One Above All stomps so bad there's no amount of training Goku could do to win. The One Above All is omnipotent. TOAA could just write Goku out of existence, not like Hakai, which takes time and can be resisted in the DBS manga. TOAA could just blink and Goku would cease to exist.
One Above All is mentioned in the Appendix glossary of the Official Handbook of the Marvel Universe, described as "the entity believed to be the supreme being/creator of the Omniverse".
Even other gods, like Thor, who acknowledge the One Above All's role as the most powerful Marvel god, find the presence of the One Above All “humbling.” Superheroes like Spider-Man and other cosmic entities have seen The One Above All.
Zeus is a God, for sure. But not the kind of god who created all of existence, no. Zeus can be defeated, the One Above all cant be. He is truly omnipotent, and thus he cant be beaten.
Saitama vs cat
[ Still trying to do the math on how, the only thing to ever actually damage Saitama in the series is a cat scratching his face.
Going by the facts offered by the last chapter, it can be argued that Goku has fought against opponents with equal or more strength than Saitama's in Dragon Ball's history. So a single Serious Punch from the bald hero would not be enough to destroy him as of yet.
At one point, the protagonist of One Punch Man even slapped himself. While he gave himself a huge red mark, he seemed mostly unaffected by it, which suggests that he could likely withstand his own punch as well.
Immeasurable Strength: Saitama possesses an immeasurable amount strength and as a result is capable of destroying powerful monsters and villains with a single one punch (though this strength is not limited to punches).
2 that it was confirmed that Stan's cameos exist for a purpose. According to a post-credits scene, Stan is actually an informant for the all-seeing Watchers, and he is seen reporting back on each of his cameos and what he saw.
Thor, the god of Thunder, was undoubtedly the most powerful member of the original Avengers. His power continues to grow in the Marvel Cinematic Universe.