The heat in the oven will slowly leak out into the rest of the house, so the home heating system of the right now already uses that. The fridge example is much more relevant though. If you could get the hot-side radiator part of the fridge thermally-connected with the outside temperature, then it would run much more efficiently. However, you'd lose that heat from the house, so if you're having to heat the house as well it might not be worth it.
But what if I don't want to heat up the home? Where I currently live I suffer from too good isolation where the kitchen heat builds up too much. I don't need the oven heat and would much rather it be piped out to the water heater.
