The standard is, so long as he doesn't stop being made of 'supercells' which function in a exact, modular fashion, and so long as his abilities conform to the laws of science, physics and nature, and so long as his cells function as cells would, it's okay. The science just has to add up.

The general rule for what constitutes as OP is as follows: Defeat of the object, being, what have you, cannot require scientific knowledge more advanced than what you'd expect to find in a highschool classroom.

Killing cells does not require a college level understanding of the sciences. Therefor, Claymoor is not OP.

That is also why my character Agatha is not OP. As long as you know the basic laws of magnetism and conductivity as taught in most high school physics classes, you'll know how she works and how to defeat her.