The main types of covalent bonds linking them are N-glycan linkage and O-glycoside. N-linked oligosaccharides are always pentasaccharides attached to asparagine via a beta linkage to the amine nitrogen of the side chain. Alternately, O-linked oligosaccharides are generally attached to threonine or serine on the alcohol group of the side chain. It is important to determine the structure of the carbohydrates present in glycoproteins. Currently, oligosaccharide analysis has been widely applied in studying the structure of the carbohydrates. https://www.creative-biolabs.com/glycoprotein/oligosaccharides-analysis.htm