Yes, they'd be fine - as long as they were over 2" when you got them, there's no way in hell an oscar could catch one lol. They need to be in groups of 5+ though, so you'd need a decent sized tank.
Oscars and SD's would be a good match as said above. Their shape helps make them not easy to fit into the mouth of the Oscar. I would recomend getting a small Oscar like maybe an inch or so big with SD's as big as you can get. The Oscar will grow faster than the SD's so just make sure they stay bigger than te Oscars mouth so that the O gets no ideas