What is the original source of energy in oil?

Answer 1

With the exception of nuclear energy (fission or fusion) all original sources of energy in the world come from the Sun and accretion (Earth's creation by impact events) . If you want to go back further than that, ultimately all energy, including nuclear energy, started with the big bang.

Oil or fossil fuels all began as plant life which gathered it's energy from the Sun, creating sugars from CO2, water and minerals through a process called photosynthesis, absorbing solar energy to do so.