Mary, Ox heart is the beef heart - you can also use ox liver (cheaper than lambs fry) or sheeps heart (as treats!) as long as you cut the white fatty bits off (the heart). Our axies never get the lambs fry, let alone fish fillets (sorry but they're my/kids treats in our house!) They, our axies!, occasionally get ox heart or ox liver (sliced in slivers - worm like!); but I'd rather feed them worms - less fuss and mess (I hate cutting raw meat...) If u can't find any in the supermarket/butcher just check at meat counter (they usually have some out back).