Depends what size the oscar is vs the barb, if your oscar is quite large and you are getting the barb when it is quite small, then there is always the possibility he could get eaten
. However, If both fish are of similar size then they make good tank mates
I have heard some reports that tin foil barbs are quite greedy and can often out compete fish for any food that is going, but I can't see it being too much of a problem as Oscars are quite the hungry little boogers as well
lol
You'll find loads of vids and blogs online were people keep oscars and tin foil barbs quite happily together