O-glycosylation is a common post-translational modification process in cell biology, involving the covalent attachment of sugar molecules to the serine or threonine residues of proteins. This process is mediated by various enzymes, including polypeptide N-acetylgalactosaminyltransferases. O-glycosylation occurs at multiple locations within and outside of cells, significantly influencing protein folding, stability, activity, and cell-cell interactions. It plays a crucial role in regulating various biological reactions and signaling pathways, such as cell adhesion, immune responses, and signal transduction. Abnormalities in protein O-glycosylation are associated with the development of various diseases, including cancer and genetic disorders.