Tbh I going to say it was not a good idea because if the Oscar tried to eat the Synodontis it will be all bad. What specie of Synodontis is it? I personally would grow it out in a separate aquarium first if it's a larger growing specie.
Its Synspilum my Oscar is pretty relaxed fish for an Oscar and been watching them and the syn will sneak up behind my Oscar and nip at his tail Oscar will spin around pretty quickly but the syn is very quick and shoot off into his rock so I'm not sure if I should keep them together
Try it and see. With the right amount of space and decor it might be fine. Fellow oscar keeper here. They can be real wimps when another fish shoves them around.
I just don't want the syn to get more aggressive as it gets bigger I think I should have done more research before I got him he picks on my pleco as well