I wouldn't put ANYTHING besides the Oscar. I had a 3 inch Oscar in my 29 gal for about 4 months. When he got to be 4-5 inches the tank started going downhill (these are messy fish and they get HUGE). I had to do something and fast to save my buddy ... so I got him his own 50 gal. He's been pretty happy in there now and in the month that he's been in there he's grown about 1-2 inches. He's fast becoming a monster! Only problem is, right now I don't have the funds to get him the bigger 75-90 gal he deserves. I'm hoping for next year, by the time he's FULL grown.
If you add your 4 inch + Oscar to the 29 gal you'll pretty much be maxed out for bioload. You can take the chance and add MAYBE 1 more fish in there (something small but big enough not to get eaten) but keep in mind that you may be pushing it and you'll definitly have to do weekly water changes of about 30-50% to keep the fih happy. Even then, you won't be able to keep your Oscar in that tank for long before he needs another upgrade. These fish need a MINIMUM of 50 gal PER Oscar when adult, 75-90 gal PER fish is better and makes for happier/healthier fish.
Hope that helps you a bit.