O-linked glycosylation is the attachment of a sugar molecule to an oxygen atom in an amino acid residue in a protein. O-linked glycosylation is a form of glycosylation that occurs in the Golgi apparatus in eukaryotes. It also occurs in archaea and bacteria. The sugar molecule attached to protein includes, O-N-acetylgalactosamine (O-GalNAc), O-fucose, O-glucose, O-N-acetylglucosamine (O-GlcNAc) and O-mannose. In biology glycosylation mainly refers in particular to the enzymatic process that attaches glycans to proteins, lipids, or other organic molecules. This enzymatic process produces one of the fundamental biopolymers found in cells (along with DNA, RNA, and proteins). Glycosylation is a form of co-translational and post-translational modification. Glycans serve a variety of structural and functional roles in membrane and secreted proteins.

Reference
Wiki: O-linked glycosylation