My oscars usual diet contists of pellets and flakes and frozen brine shrimp. He is now big enough to to eat larger live foods. How often should i feed these to him and what would be my best choices? He seems to like guppys and ghost shrimp

Shrimps, crickets, mealworms, earthworms, that would be my choice.

As for how often, live food has all the nutrients and vitamins that fish need so if you wanted to rotate this type of food, I don't see a problem at all.

be careful with live fish, quarantee them for 2 weeks to make sure they have no diseases, or even better breed your own

It is not neccessary to feed live fish to your oscar and i personally dont do this as i think its cruel however lots of people do feed live fish so if you do, dont do it too often (once a week or once every 2 weeks) as an oscars diet in the wild is mostly insects etc. You can see from an oscars shape that they are not designed mainly to eat fish, otherwise they would be torpedo shaped

There are lots of live foods you can feed your Oscars, try looking in your garden for insects snails and worms they are great food and they are free.

yup becarefull of feeders and dont use any worms ettc from the garden that fertizlers,pestisides etc have been used on. Rotation is the best thing,,, best thng is breed your own feeders (like convicts for eg)