Oligosaccharyl transferase is an enzyme that catalyses the transfer of an oligosaccharide from a dolichol-linked precursor to an asparagine residue in a nascent polypeptide chain. This process, known as N-linked glycosylation, is essential for the proper folding and function of many proteins.

The oligosaccharide protein transferase complex is composed of several subunits, including the catalytic subunit (OST-A), the regulatory subunit (OST-B), and the accessory subunits (OST-C, -D, and -E). The catalytic subunit is responsible for the transfer of the oligosaccharide from the dolichol-linked precursor to the asparagine residue, while the regulatory subunit controls the activity of the catalytic subunit. The accessory subunits are involved in the assembly and stability of the complex.