This is false. But for the OP Oscars and LMB. I would suggest pellets being roughly 80% with nightcrawlers and Crickets being 2 very good options for live foods. Oscars being particularly sensitive to HLLE/HITH due to thiasmine issues ( which is why goldfish and rosy minnows are discouraged in general other then the disease/over all health of these fish)
I would avoid frogs unless you are 100% sure of their health and species as some are toxic. amphibians skin will "soak" up any toxins in the waters they come from, which can and will be passed onto your fish.
crawdads are acceptable as long as they are captive raised/propigated as well for poison concerns.
There is some great info stickyed on this site for further info.