Why is there a smell of oil when the car heater is turned on?

A smell of oil when the car heater is turned on could indicate an oil leak that is coming into contact with hot engine components, such as the exhaust manifold, producing a burning odor. It may also be caused by oil residue on the engine that heats up when the heater is activated. Additionally, if the heater core is leaking coolant, it can mix with oil and create an unusual smell. It's advisable to have the vehicle inspected to identify and address the issue promptly.