Electron transport and oxidative phosphorylation occur in the matrix of the mitochondria.
To elaborate, the mitochondria are often referred to as the powerhouses of the cell because they produce ATP, the energy currency of the cell. Within the mitochondria, the electron transport chain is located in the inner mitochondrial membrane. As electrons are transferred through various protein complexes in this chain, a series of redox reactions occur, ultimately leading to the pumping of protons into the intermembrane space, which creates a proton gradient.
This gradient powers ATP synthase, an enzyme that synthesizes ATP from ADP and inorganic phosphate via oxidative phosphorylation. Therefore, while the whole process is intricately connected within the mitochondria, the actual mechanism of oxidative phosphorylation takes place in the inner mitochondrial membrane, with the reaction substrates and products largely associated with the mitochondrial matrix.