Black gooey stuff isn't a 100% guarantee Oscar has ear mites, though.
I was absolutely sure Yoda had
ear mites once and it wasn't. The vet needs to look at his ears
through an otoscope to be sure.

I agree. The best way to get a diagnosis is to see a vet. Don't buy
over the counter ear mite treatments--they don't generally work.
Please have this looked at. Left untreated, ear mites can cause
hearing damage, not to mention being extremely uncomfortable for the
cat.