I have physical retail copies of Oblivion (regular and GOTY editions from 2006 and 2007) and with the latter I have no problems installing mods and OBSE.

I bought the GOG Ultimate Edition a while back and on that install OBSE refuses to work. That is, the game loads to the menu and I can choose to start a new game but towards the end of the load-up the game crashes to desktop. Tried a couple of fixes (there's only so much you can do) but to no avail.

I was wondering if there's a possibility to mix both installs. For instance, install Oblivion GOTY from DVD and then let GOG Galaxy perform a file check, in the hopes that this mix would let me run the game from the OBSE launcher...

GOG Oblivion and OBSE work together (at least on my PC). Try without Galaxy?

Uninstalled the GOG version and re-installed my GOTY DVD version that can be added to Steam but it doesn't get updated through Steam so I was missing all the extras since the DVD GOTY has only the Knights and Shivering Isle DLC.

Installed the GOG edition again and whaddaya kno, this time the game launched with the OBSE exe.