Yep we just need the tank size ie, gallons, so we can advise which fish you might be able to add, obviously we wouldnt suggest certain or any fish if the oscar was in say 180L tank (not saying thats what you have!). But if you had a 200 gallon tank then the situation would of course be different and you could have a choice of fish.
it is a 70 gallon fish tank
Well in certain circles, 75gallon is recommended for one Oscar alone, so I cant recommend any other cichlids as companions Im afraid, however you could get some dithers to liven the place a little, such as silver dollars or tinfoil barbs maybe, but depending on the size of the Oscar, you would need to get these already close to full grown, so they cant fit in the O's mouth. However saying that, if your O is already large, he will see the entire tank as his space and will probably object strongly to any tankmates.