Oligosaccharides are linked to proteins by O- or N-glycosidic bonds
In N-linked oligosaccharides
N-acetylglucosamine at the C1 position of reducing end of sugar binds to Asn within Asn-X-Ser/Thr sequence.
O-glycosidic bonds are formed between C1 position of reducing terminus of monosaccharide or oligosaccharide and hydroxyamino acid, Ser or Thr.
(N-acetylgalactosamine is often at the non-reducing end.)