Was just curious, I have a fully modded install of oblivion GOTY, but have since lost/misplaced the discs. I recently repurchased the GOTY edition on Steam since it was fairly inexpensive. Would it be possible to just copy/paste over my Data folder, or would that cause any mod related strangeness? Not really looking forward to reinstalling all my mods/Wrye Bash etc.

Try it.
I recently moved from Retail DVD(s) to Steam version for the same reason, the only difference should be the .exe files.
Make a backup through Steam/Windows if you have it installed, then copy over the other install, then copy back the Steam .exes or use the Verify Integrity function.
Verify would be disasterous if any mods changed default files (optimised .BSAs, etc), you can see what files fail and are reacquired from GameValidation.log in the Steam folder if you dare try it!

Be aware that you HAVE to enable Steam in-game community if you want to use OBSE, something I had disabled on general principle, took me a while to work out why it wasn't working.

Good luck