No Oscars with other fish; Oscars get HUGE and it would need a minimum of 55 gals just for one without anything else.
It's hard to find tank mates for Red Tailed Sharks as they can be aggressive. You could do a shoal of maybe 12 Tiger Barbs. I'd watch the Shark and Pleco though as they both inhabit the bottom levels of the tank, the shark may eventually go after the pleco. I'd upgrade as soon as you can; a shark and a pleco in a 10 gal is a recipe for disaster.
As for the 10 gal, you have some options. You could do:
a single betta OR
two dwarf puffers OR
a shoal of 6-8 nano fish like ember tetra, chili rasbora or celestial pearl danio OR
2-3 platys OR
3-4 guppies