Space

I'm not entirely sure what you're getting at. Are you saying that it's possible life could produce energy from non-oxygen sources? If so, then yes, that's possible, but because such organisms don't exist in large quantities here on Earth it's hard to predict what kind of gases they would produce so that we can look for them.

However, we know that if we detect free oxygen in the atmosphere in huge quantities and conditions are favorable for life on that planet like liquid water, well... There are no known natural phenomenon that produce huge amounts of oxygen because oxygen loves to bond to other chemicals, so if there's oxygen then something made it by breaking oxygen from it's bond with something else and recently.

Obviously we could look at a planet, see no oxygen, and assume that there's no life and be wrong, but if we see a lot of oxygen in the atmosphere and conditions are favorable for life then it's pretty much confirmed that there's at least microbes producing that oxygen.