Forget oscars if you will have any other fish in with it. The minnimum for a lone oscar is 55 gallons. The fact that they are currently small plays absolutely no role as you should consider their adult size (which they reach very quickly if cared for well) and the amount of waste these messy fish produce - not to mention their aggression and territoriality.
The shark you have will grow to 5-7" in time and your tank will slowly fill out even without some huge aggressive cichlid in there. Anyway, if the shark's a rainbow shark all its fins should be red. If it's a red-tailed black shark, only its tail should be red and its body much darker black than that of a rainbow. Rainbows tend to grow ever so slightly larger but are actualy a little less aggressive. It doesn't matter in the end though - you can't keep more than one shark-type fish in the tank regardless.
4 tiger barbs is not enough - start filling out the tank by getting at least 3 more.
Then start thinking about further tankmates. If you are still sticking to the cichlid idea, ask in the relevant forum. I personaly would get a pair of kribs/pulcher as a cichlid addition to this boisterous/semi-aggressive tank but a severum would also likely work.
Beyond that I don't know but keep in mind that the tiger barbs are a nippy fish so nothing long-finned. Also watch your bottom-dwellers (if you add any catfish, loaches etc) as kribs will chase them around and don't add anything small as a severum would eat it in time. Having said that, one bottom-dweller to consider as an addition would be the bristlenose plec or similar small-growing plecos and if you don't get a severum anything that won't get attacked by the barbs or swallowed by the shark should work out fine with kribs.
Mollies and swordtails also usualy work (no fancy fins though), as do large danios (giant for example), boisterous barbs (rosy, ticto, half-banded, golden, odessa, five-banded, ember, black ruby, spanner etc) and some of the more active rasboras and larger tetras (black widows/skirts, sometimes serpae tetras, brilliant rasboras, elegant rasboras, giant scissortail rasboras etc). Some of the larger rainbowfish (glossolepis incisus for example) will also work most of the time. Make sure any schooling fish are kept in adequately-sized groups. Also, watch that any livebearers you get are kept in the correct gender ratios. Avoid anything particularly slow, small or long-finned.
Someone has suggested keyholes or severums to me - maybe you should check those out...
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