Nuska's Oblivion Character Overhaul 2 is just amazing, as I'm sure many of you would agree. And while Morroblivion's characters look perfectly fine with this installed, I've decided that I want to go through and change at least some of the more memorable characters for my playthrough so that they more closely match their MW appearance.
I'm still in the stages of researching and gathering/creating some resources that OCO2 doesn't have (or at least that I haven't noticed), mainly facial scars and eyes and whatnot.
I'm still pretty much a beginner at modding (I've only ever done simple retextures and have created a personal race or two) and I'm not particularly versed in the CK yet—though I imagine this will give me plenty of experience as I try and fail and try again. So any advice you may have to share is very welcome—and probably needed.
I do have a question though, which is why I've decided to post here before anything has yet been done. Which Morroblivion main files should I use as masters? Both the esm and esp?
Thanks

You only need Morrowind_ob
You only need Morrowind_ob.esm and Oblivion Character Overhaul.esp as a masters. Morrowind_ob.esp only contains a few overrides and is mainly used to load BSAs.
If you're not already using CSE, grab it. It makes modding an enjoyable process, rathet than a struggle with vanilla CS.
Other than that, making a patch is mainly going through Morrowind's NPCs and changing their facial features to match OCO style.
You probably don't want to spend too much time on Vivec and Almalexia, as these two will be getting a new look eventually anyway.