OCO tends to mess up faces of NPCs added by mods because, well, it messes up faces. That is, faces need to be redone to look good with OCO. You may want to check whether there are OCO patches for the mods you want to use, if you are looking to install quest mods in particular. You'd notice those NPCs more than some random bandits.
There are also a number of new mods that modify vanilla heads by gently changing the mesh. They are designed to work with vanilla faces, that is, they don't deform them. The results are quite impressive! It is suprising how little the mesh needs to be changed for the face to look different.
For example, see this:
Undeformed Potatoes - fixes up vanilla NPCs without changing the mesh.
Enhanced Vanilla Faces - changes the mesh slightly which improves vanilla faces. Works well with Undeformed Potatoes.