You can train them to accept any food. There is nothing the guppies can eat that the oscar can't. And there is no benefit of using feeders.
I have real predators, that generally only eat live foods, and I have them all eating dry foods. It was a long process to get them off of live foods, but it happened, and they're all very healthy and happy.
Remember, if you do use feeders, theirs a chance they will stop eating dry food and only want feeders.
Not to mention the cost of feeders. Even if you breed your own, you will need another tank, another power filter, and food for them. All thia added cost when you could have just got a bag of pellets instead.
And since the tank your Oscars are in is already too small, I doubt you have the means to set up a proper breeding tank.