A reader pointed out that the right tag for my MC would be strongMC and not OP. Now, that might be true, but
1) I can't seem to find the strongMC tag
2) I made a different reasoning
So, Reidar has a very peculiar power, which turns him stronger the more allies are around (essentially) this makes it so that Reidar is easily several levels higher than the monsters in the surroundings, which means being several levels higher than the rest of the people.
Several levels mean a lot of levels. At some point, while most of the people are at level 30, Reidar is at level 70. This means Reidar actually is OP.
But him being OP doesn't mean there are not other OP beings. Reidar will in fact fight against things that would not normally be possible to face. I don't know what your definition of OP MC is, but in this case here is a relative OP MC.
He is OP compared to humans and most monsters, but he isn't the only one, and he isn't the only thing. This allows me to keep tension and create conflict inside the book. Hope this explains it.