It's really easy.
First, go to your c:\Users\user_name\Documents\My Games\Oblivion\ folder amd set oblivion.ini to read only.
Second, create a simply txt file with Notepad
SetINI "fDefaultFOV:Display" 90
and safe it as 90.txt in your Nehrim directory (the same location where the Oblivion.exe is stored).
Third, start Nehrim and load a save game, type ^ for the console, type bat 90.txt Enter and again ^.
Now save up, at the next Nehrim run load this save game and you have automatically FOV 90.
Finish.
Do not change the FOV directly in the oblivion.ini, this will generate strange mouse vs. menu dis-adjustments.
Create a second txt file with fov 100 and save it as 100.txt, for comparing, and so on ...... ,
or type this darkblue command under "Second" directly, not case sensitive, in the console.