How much energy we get from oxygen?
I know that you already picked an answer, but I found an alternative answer for you. On average a person consumes 550 litres of oxygen in a day. Now, our body could burn carbohydrates or fats or proteins to produce energy. Each litre of oxygen can produce 5.04 kcals if it burns carbohydrates, produce 4.68 kcals if it burns fats or produce 4.48 kcals if it burns proteins (reference). That being said, if I consider that all 550 litres were used to burn carbohydrates, a total of 550 x 5.04 kcals will be produced which is 2780 kcals.