Can I reuse the oil from a can of sardines/anchovies for cooking?
Yes, as long as the oil hasn't spoiled. The trick will be to try to pair the oil with something that goes well with a fish flavor because the oil will have the taste and aroma of the fish that were floating in it. If you're cooking fish in a skillet then that might work well. If you're vegetables on the other hand you might end up with broccoli that tastes like fish.
It also could be used when creating spreads that are meant to be paired with fish.