A random seed getting into the wrong packet is a somewhat common happening. An accidentally cross-pollinated seed in a packet is even more common. And sometimes fruit shape can differ slightly on the same plant depending on air temps at pollination time. So several possible explanations. Just enjoy the tomatoes and don't worry about it.

As to the variety name - there are several "oxheart" tomatoes. Do you know which one you have specifically? What was the source? Oxheart 4153 is normally a round ribbed fruit. Japanese Oxheart has a pointed nipple though it is less obvious on some fruit. Then there is one called Upstate Oxheart that is heart shaped but with no nipple end.

Dave

To my understanding, each seed is a different entity like babies grown in the womb of the plant which is tomato here. Each is produced by a different ovule and different pollen. Like a mammal that can have multiple birth that not all of them necessarily are identical. Of course human error in collecting and packaging is a stronger possibility for cases like yours.

Seysonn

Regardless of the reason occurances like you experience can be very frustrating. Last year I tried a new variety called Grandpa Dutches Favorite from a seed Co. I'll not mention. Each of the 4 plants I trialed produced drastically different fruits; one even turned out to be a cherry type. I now have come to expect 5-7% "non-typical" plants in any heirloom variety planted. If I see 1% "non-typical"with hybrid varieties I'd be extremely surprised.

There may be an occasion that you get different color scheming such as with non-segregated crossings; Copia is a good example. Those are more tolerable and might even be looked on as pleasant surprise. I plant thousands of tomato plants and sell buth plants and fruit and I get concerned about other's results as well as my own. If you only plant a few plants those oddball ones can be deal breakers.

I grew Red Robin for the first time last season. One plant was absolutely wonderful-great little tomatoes with a beefsteak flavor. But one plant turned out to be cherry sized fruits that were deeply pleated and very sour. So now I'm paranoid about wasting my efforts on Red Robin seed which I really want to grow regularly.