During a whole day breathing, how many kcal or watts do we get from the oxygen we breathe?
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.
This question is not actually answerable.
Consider this. Take a nice campfire. It's warm and toasty. How much energy comes from the wood you piled at the bottom. It's a lot right?
Now how much energy comes from the oxygen the fire consumed?
We know that a fire needs three things to burn: fuel, oxygen, and heat. How much energy comes from the heat?
Who creates a baby, the mother or the father? Obviously the mother caries it to term, but the creation itself takes two, interacting.
These questions don't have clear answers because there's no natural division to be made. We can't state how much energy came from the fuel or how much energy comes from the oxygen, because it is not the fuel nor the oxygen which provides the energy, but the reaction between the two.
In general, we find oxygen is more accessible than fuel. We can always get oxygen from the air by breathing, but one has to find oxidizable fuel (food). As such, we find it convenient to associate all of the energy of the reaction to the fuel, for bookkeeping purposes. But that's just bookkeeping. We do this because, if we're short on oxygen, we just have to breathe a little more. If we're short on fuel, we have to go find fuel (food).
Oxygen does not provide any calories (energy) or watts (power), just the same as a water bottle states it contains zero calories. Energy is held within glucose, or other macromolecules than can be broken down to supply electrons that generate ATP, or other similar molecules.