The aggregation, arrangement and bonding together of a set of components to form the oxygen evolving complex (OEC) of photosystem II on a thylakoid membrane. The OEC protects the calcium-4 manganese-5 oxide cluster which is bound to the D1 and CP43 proteins. The exact protein composition of the OEC varies between cyanobacteria and plants, and in plants consists of three extrinsic nuclear-encoded polypeptides: PsbO, PsbP and PsbQ.