There are lots of techniques, I think, that can help. Take, for example, shuffling the load around. When translating I often encounter situations whenit is impossible to convey all required nuances in the same paragraph of the character's speech - say, the target language just doesn't work that way. I then simply shift some of these nuances to his adjacent phrases, thus keeping the overall tone and impression.
You may try finding such bottlenecks in your plot and shift charaters' in-detail introductions around (kind of like forward declarations in programming languages) -- you first show the characters briefly through someone not pointing attention to them, or use "tell, not show", then introduce that person later when you have room.
OR, do the contrary and bloat it even more -- with plot, so that these characters get room to be introduced properly. Use filer side stories and such
How many OCs you have and how many chapters? Considering that introducing 2 OCs per chapter is already pushing it?
Also, we readers need breathing room to get used to new characters after they are introduced, without more being added immediately.
I am now trying a new technique I invented (like a new bicycle, heh): Double Impact. I first describe that character's appearance in broad strokes, then add some confrontation involving them -- action! -- then comes the second round of more detailed description in more calm situation through the eyes of other characters and their opinions.