You should have removed the cover and see where it was leaking from; it takes me a couple of minutes with the car already lifted. Could be something as stupid as leaked fluid when filling the transmission, or something serious that requires removal to fix. Only an inspection can tell, so forget about speculation, as with the cover in place, oil could even be from the engine (you didn't even tell us where the oil was; it's a large cover). Just take it in and wish for good news
. Good luck.