most catfish get fairly large. a pictus would be fine for a while, but they only grow to 5 inches or so, so that would eventually become food (more likely just get stuck in his throat). plecos i dont reccommend at all for oscars as tehy can make a nasty hole in your fish in the night. off the top of my head, i would say a lace syndontis. he grows to about 7 inches, which is just about perfect companion for that tank with a full grown o in it. they may look tiny in it today, but in a year, that tank will look just right.
heres a pic.