O-linked glycosylation is a biochemical process where sugar molecules are attached to the oxygen atom of specific amino acids in proteins. This modification typically occurs on serine or threonine residues and plays a crucial role in protein stability, signaling, and cell recognition. This type of glycosylation is essential for the proper functioning of many proteins, including those involved in cell communication and immune responses. O-linked glycosylation is commonly found in mucins, which are glycoproteins that protect and lubricate surfaces in the body, such as the respiratory and gastrointestinal tracts.