Oxygen is converted to carbon dioxide in the cells, which essential burn (oxidize) glucose or fat (a bunch of carbon) for fuel. Fat plus the oxygen you breathe becomes carbon dioxide and water. That's why aerobic exercise make you thinner (you burn your carbon sources and exhale the weight)

The process is called osmosis. There is a thin membrane that separates the air in your lungs from the blood in your lungs. The blood has a high concentration of carbon dioxide and a low concentration of oxygen. Any volume of fluid (liquid or gas) wants to equalize, so the carbon dioxide naturally tries to reach the low concentration air while the oxygen tries to reach the low concentration blood. The fun part is your body doesn’t actually know if it has enough oxygen, it only knows how much carbon ioxide it has. So in for example low oxygen, high nitrogen air you can suffocate without noticing since the carbon dioxide is still leaving your body.

https://en.wikipedia.org/wiki/Respiration_(physiology)

Glycolosis and the Kreb's cycle. The main difference between glycolysis and Krebs cycle is: Glycolysis is the first step involved in the process of respiration and occurs in the cytoplasm of the cell. While Krebs Cycle is the second process of respiration which occurs in the mitochondria of the cell. Both are the process involved in respiration with the aim of fulfilling the energy requirement of the body.

So Glycolysis is defined as the chain of the reactions, for the conversion of glucose (or glycogen) into pyruvate lactate and thus producing ATP. On the other hand, Kreb's cycle or citric acid cycle involves the oxidation of acetyl CoA into CO2 and H2O.

Respiration is the important process of all the living being, where oxygen is utilised and carbon dioxide is released from the body. During this process, energy is released, which is used to perform various functions of the body. Apart from the above two mechanisms, there are various other mechanisms of respiration such as the electron transport system, pentose phosphate pathway, anaerobic breakdown of pyruvic acid, and terminal oxidation.