We had what was described as a hybrid furnace in our last house. The heat pump would cool and heat above a fixed outdoor temperature. Perhaps 15f. Below that, a conventional oil burner would fire. The changeover temp was based on the efficiency cross over point of the two fuels. I live in New Hampshire about 50 miles from the ocean. The oil consumption was perhaps 20% of what it had been on full oil.

I am adding a mini split in my current home to supplement the existing oil burner. (Have solar).