Glycosylation is a form of post-translational modification of proteins and occurs in every living cell. The carbohydrate chains attached to the proteins serve various functions. There are two main types of protein glycosylation: N-glycosylation and O-glycosylation. In this paper, we describe the O-glycosylation process and currently known congenital disorders of glycosylation associated with defects of protein O-glycosylation. This process takes place in the cis Golgi apparatus after N-glycosylation and folding of the proteins. The O-glycosylation is essential in the biosynthesis of mucins, the formation of proteoglycan core proteins and blood group proteins. Most common forms of O-glycans are the mucin-type glycans. There are more than 20 known disorders related to O-glycosylation disturbances. We review 8 of the following diseases linked to defects in the synthesis of O-xylosylglycans, O-N acetylgalactosaminylglycans, O-xylosyl/N-acetylglycans, O-mannosylglycans, and O-fucosylglycans: multiple exostoses, progeroid variant of Ehlers-Danlos syndrome, progeria, familial tumoral calcinosis, Schneckenbecken dysplasia, Walker-Warburg syndrome, spondylocostal dysostosis type 3, and Peter's plus syndrome. Causes of these diseases include gene mutations and deficiency of proteins (enzymes). Their diagnosis includes syndromic presentation, organ-specific expression and laboratory findings.