Attachment of sugars to oxygen atoms of amino acids

O-linked glycosylation occurs in the ER/Golgi apparatus in eukaryotes. It also occurs in Archaea and Bacteria. O-N-acetylgalactosamine (O-GalNAc) may be attached to serine or threonine residues - the high concentration of carbohydrates gives mucus its "slimy" feel. O-mannose can be attached to serine and threonine residues in secretory pathway proteins. O-mannosylation is common to both prokaryotes and eukaryotes. O-fucose and O-glucose can be added to cysteine residues.