Substrate level phosphorylation is the attachment of a phosphate onto a molecule by another molecule. For example:
GTP + ADP ---> GDP + ATP
Oxidative phosphorylation is whereby a phosphate is attached onto ADP by means of harnessing the energy produced from a proton gradient that was established by previous oxidation reactions, e.g. ATP Synthase phosphorylates ADP to ATP by a proton gradient in the intermembrane space of the mitochondria.