NDTV television channel reported 14 more people were injured in the early morning incident near Manor on the Mumbai-Ahmedabad highway while the passengers slept.
Emergency crews and police were at the scene, the report said.
Authorities warned of risks of explosions from the spilled diesel.
The Indo-Asian News Service quoted police as saying preliminary information showed the collision occurred when the driver of the luxury passenger bus may have attempted to pass the oil tanker.
The report said nearby villagers assisted firemen in putting out the fire.
Two of the injured victims were reported in serious condition at a nearby hospital. The condition of the other injured passengers was not immediately known.