That explains why my butt hole burns sometimes

Not your belly, but in your cells, yes.

The reason you need oxygen, is that your cells use it to “burn” stuff a few atoms at a time, for energy.

It’s the same reaction as fire. Oxidation. Though inside our bodies it happens in an extremely controlled manner.

Although cellular respiration is technically a combustion reaction, it is an unusual one because of the slow, controlled release of energy from the series of reactions.