I am traveling to Frankfurt, and I have to do self-transfer at Oslo. I will be applying for a Schengen visa for Germany, but do I need a transit visa for Oslo airport as well? I will be getting on a flight from the same airport, but I don't know if they will make me do customs again. I have an Indian passport btw.
No, you do not need a separate visa. Norway is a member of the Schengen Area, so your Schengen visa entitles you to transit through Norway on your way to Germany.
You will clear immigration and enter Schengen at Oslo. Your onward flight to Frankfurt is within Schengen and effectively "domestic", meaning no passport control.