Is energy in petroleum or in oxygen?

BOTH

the energy is stored in the electon configuration of the two molecules or atoms

the chemical reaction is a rearrangement of the electrons to a diffenent configuration

if energy is released we call it burning or "exo thermic"

if we have to provide heat or pressure to make the reaction happen it is "endo thermic" takes up heat.

the ancient sun energy and life processes in the cells force the low energy O2, C and other componds into higher energy carbohydrates which could later oxidize back to lower energy levels

chemical reactions change electron configuration

Nuclear reaction = change the mass and nucleus of atoms E = MC^2 , tens of millions of times more energy level change

You are right. The reaction between the petroleum and the oxygen releases energy, but you need both. We take oxygen for granted, and so we say that the petroleum is the fuel. But on Titan, you would take the hydrocarbon for granted, and if there were a source of oxygen, that is what you would regard as the fuel.

In the same way, ATP does not contain energy. ATP + water releases energy on conversion to ADP + inorganic phosphate, but we take the water for granted.

Everything has energy

If you go back to chemistry you learn that when bonds form, they release energy. When they break they absorb energy. This is because when they form, they get more stable and therefore have less energy. So whatever your reaction is, if it forms something new you are most likely to get energy in form of heat. Unless it is an endothermic reaction to which it absorbs heat