i wont go into a long post about live food, a sim sure you get the idea. but if you do want to give live food as a treat, or as stimulation for them and entertainment for you, your best bets are ghost shrimp, earthworms, or crickets. all great for your fish as a treat 1-2x a week, yet not a huge chance of passing any disease off.
now, you have a 55g, which chances are wont even be big enuff for one oscar once he is full grown. a typical 55g is 12" wide, and oscars commonly reach that 12" mark and even an inch or two larger. the bare minimum for one oscar would be a 75g for life. that being said, an oscar will take a while to get to that full size, but it will happen eventually, and he'll need atleast an 18" wide tank. you can keep him in a 55g for now, but know you will almost definatly need a bigger tank.
as for sexing, im pretty sure theres no way to sex an oscar 100% without venting it, which can be extremely stressful to the fish.