In general no, it's not safe. This is because a lot of feeder fish (no matter which kind they are) may carry diseases. These diseases can then be passed on to the fish your feeding them to.. So in this case, your Oscar.
Also there is no reason to feed live fish anyway. There is no nutritional value in feeder fish for your Oscar and will do more harm than good. For the best results feed your Oscar a good quality cichlid pellet, hikari and new life spectrum are both good choices. Then you can occasionally feed him/ her things like frozen fish (smelt), frozen bloodworm cubes (or live bloodworm), shrimp, krill and frozen market prawns. Pellets should be fed daily (or twice daily) whilst the other things iv mentioned are a once or twice a week treat.
Iv owned lots of big cichlids like oscars, jaguars, dovii's, red devil, red terror and so on and know I have a flowerhorn. Iv also had red belly piranhas, birchirs, snakeheads and Lima shovelnose catfish... Never have I had the need to feed any of them live fish and all have grown very well with no problems