The most impoant factor controlling the rate of mitochondrial oxidative phosphorylation is the:

Respiratory control refers to the regulation of oxidative phosphorylation by the cellular ADP levels. Electrons do not flow through the electron transpo chain to O2 unless ADP is simultaneously phosphorylated to ATP. This only occurs when there is a need to synthesize ATP. The rate of O2 use increases significantly in experiments where ADP is added to isolated mitochondria. Oxidative phosphorylation requires NADH, O2, ADP, and Pi, but ADP is the most impoant in terms of regulation. ADP increases as ATP is utilized, therefore, high ADP indicates low energy charge in the cell. Ref: Botham K.M., Mayes P.A. (2011). Chapter 13. The Respiratory Chain & Oxidative Phosphorylation. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.