I've got a mesh replacer for the Arena heavy raiment. None of that skimpy nonsense, just reshaped to fit the HGEC body. Unfortunately, in-game it still shows the old mesh. I know the new mesh has more substance around the calf area. I've checked the meshes folder, and the correct mesh is there, and according to TES4Edit no plugins are modifying the 00029921 form (none that I've found, anyway). It's possible other armours are screwed up in this way.

I've run Archive Invalidation in OBMM (Utilities -> Archive Invalidation -> Update (BSA redirection). Is there anything else I can try?

EDIT: Just checked the timestamps, the Oblivion BSA files are set to 3rd April 2014 (when I reinstalled the game) and the mesh to sometime last year.

EDIT2: Copied the BSAs from my old Oblivion install that I backed up just in case anything broke on my new one, and it's working now. I blame Steam - yet again it's proven that it can't do simple things without screwing up in a spectacular manner.

The timestamps need to be reset regardless of whether or not Steam is in use.