How do oven mitts keep the heat out?

The usual design of oven mitts is with multiple layers of heat-resistant cloth. The air spaces between the layers reduce the amount of heat that can be conducted directly from the heated materials. Much of the heat is lost to the air rather than allowed into the interior of the glove.

This is similar to the function of a blanket, but in reverse: a blanket holds in body heat, while an oven mitt holds in cooler air.