Substrate-level phosphorylation occurs during Glycolysis and the Kreb's Cycle and involves the physical addition
of a free phosphate to ADP to form ATP. Oxidative phosphorylation, on the other hand, takes place along the
electron transport chain, where ATP is synthesized indirectly from the creation of a proton gradient and the
movement of these protons back accross the membrane through the protein channel, ATP synthase. As the
protons pass through, ATP is created.
Reference: Lambeth, Frohlich, Milavetz, BI (2004). "Expression of two succinyl-CoA synthetases with different nucleotide specificities .. The Journal of Biological Chemistry279 (35): 36621–4. doi:10.1074/jbc.M406884200. PMID15234968