Hello Flying Kites
can you send us a saved game?
You can find it at: ...\Steam\steamapps\common\Oblivion\Saves
Game saves always are made of 2 files, a -.ess and a -.skse file with the same name, put them both in an archive and upload this here as an attachment (zip, raw).
Are you using any mod´s? If yes, test it without mod´s.
What else you could try is to repair the installation via Steam.
Note: set you first always have to activate Oblivion with the switcher, otherwise you will destroy the installation of both games.
I just saw your other post, are you using the Oblivion version from GOG or from Steam?
With the gog version you would have to use the older Nehrim version, then of course the repair via Steam is not possible.
(preferred launcher setting see image)