Oxygen in it's purest form is a deadly gas that would kill is almost instantly. It is the cause for most (if not all) cases of combustion. We breath in two molecules of oxygen, which stabilizes the compound and allows us to dispel carbon dioxide safely. It only comprises a small part of the atmosphere though. The air is made up of around 93% nitrogen, which is vital for the survival of certain species on earth. There is also a carbon cycle in the air which will be destroyed, given that only oxygen was in the atmosphere.
If your carbohydrate consumption was low, then the process of making ATP would quickly diminish. Carbohydrates are the main source of energy to cells, and it's because of them that we can make ATP.
This would affect your cells by slowing them down, which ultimately slows you down as a whole. If it gets too serious, death can occur.
