I shouldn't worry about the Oscars being too shy, you need to hang around the tank a bit more and let them get used to you
Oscars are often very fussy eaters. I'm afraid that I can't give you a miracle cure to make your fish eat whatever you want them to. Sometimes they just will not eat a certain type of food. Oscars don't get hungry like mammals do so they can go for long periods of time without eating.You could try starving them for a few days and see if they will accept the food, if that doesn't work then you will just have to keep trying food until they find something they like.
Personally, I wouldn't bother with beef heart, this type of food is okay if you want to grow young fish, but there really isn't much nutritional value unless you are creating your own food packed with lots of vitamins etc. Even then, I'm not keen on feeding animal meat to fish, even though it is quite lean.