In the 2nd stage of the kreb cycle with oxygen present,
glucose breaks down into two molecules of pyruvic acid
glucose is broken down into three molecules of pyruvic acid
pyruvic acid is broken down into carbon dioxide molecules, water, and energy
oxygen is combined with hydrogen atoms of nadh to make water and energy

Oxygen is combined with hydrogen atoms of NADH to make water and energy

In the first stage of Krebs Cycle or Citric Acid Cycle, 2 carbon acetyl group bind to oxaloacetate to form citrate. Citrate will then be converted into isocitrate with the help of isocitrate aconitase. In the next stage, the two citric acid molecules formed will be oxidized (combined with oxygen). This will produce hydrogen atoms that will bind to NAD and FAD, producing NADH and FADH2.