An oligosaccharide is any glycan built from less than 20 monosaccharidesunits and connected by glycosidic bonds. Similar to amino acids and protein or nucleic acids or nucleotides, monosaccharides relate to oligo- and polysaccharides as their single-unit building blocks. Many oligosaccharides are component of glycans and connected to non-carbohydrate elements (Seeberger 2015-2017).