Which amino acid in a protein acts as a potential O-glycosylation site for attachment of an oligosaccharide unit?

O Glycosylation occurs between OH group of the amino acid and oligosaccharide, while N glycosylation occurs between the amino group of the amino acid and an oligosaccharide chain.Tyrosine, serine, and threonine are OH group-containing amino acid which gets attached with the oligosaccharide chain with the help of O glycosidic linkage. Glutamine, aspargine are amide group-containing amino acid which gets attached with the oligosaccharide chain with the help of N-glycosidic linkage.(Ref: Harper's, 25/e, pg 682)