As you can see by the responses, there is no general rule. But if your Oscar has been in the tank a while and is territorial, it may not accept anything you put in with it. It sometimes work when all fish grow up together from a young age, but when they are adults, it is always a gamble with any cichlids. Make sure the pike you get comes from/accepts the samewater conditions, there are some pike species that come from temperate S America and appreciate cooler temps, or a cold seasonal change.