Thanks.
Well, it is a bit more complicated than simply having added mods, but yes, I have made some changes.
(long version):
1. I use Mod Organizer for modding Oblivion.
2. Another mod I have required a bashed patch to work properly, so I installed Wrye Bash and built one (through MO, as necessary).
3. When building the patch, the Vilja Coolsims Hair could be, and was, merged in it.
4. This setup seemed to work normally, or at least I hadn't come across any visible problems.
5. A few days ago I installed Nehrim.
6. (this may or may not be related to the issue, but I will include it for completeness) Somewhat by momentum, I also installed Mod Organizer in the Nehrim installation (uses the Oblivion engine, after all) - it turns out, however, MO was only meant to be used with 4 specific games, and did not detect Nehrim; instead, it set itself up to work with my Oblivion installation, for which I already had a copy of it in place. I uninstalled the second MO copy.
7. The oblivion.ini file went missing from the /Username/Documents/My Games/... directory.
8. Starting Oblivion and loading a save file (with the bashed patch activated), Vilja had her oiginal hair, as if the hair mod wasn't installed at all.
9. Activating the ViljaCool esp (just in case - once mods are merged into the bashed patch, their esps are deactivated) seemed to bring back the modded hairdo.
10. Rebuilding the bashed patch didn't (seem to) change anything.
11. -Deleting- the bashed patch, and building it again, the cool hair is back with, ahem, a vengeance - it is now double
). (For all I know, there could be other problems, but because this mod is cosmetic, it is immediately visible, so I came across it first.)
I use BOSS for the load order, and I don't think I have made any changes in either order or adding/removing mods except the above; the save file(s) getting corrupted during my trying things on and off is not impossible, but I am not sure how to test for it.
Hopefully, this should help narrow things down somewhat.
Should any further information be necessary, please let me know.