An oligosaccharide (/ˌɒlɪɡoʊˈsækəˌraɪd/; from Ancient Greek ὀλίγος (olígos) 'few' and σάκχαρ (sákkhar) 'sugar') is a saccharide polymer containing a small number (typically three to ten) of monosaccharides (simple sugars). Oligosaccharides can have many functions including cell recognition and cell adhesion.

They are normally present as glycans: oligosaccharide chains are linked to lipids or to compatible amino acid side chains in proteins, by N- or O-glycosidic bonds. 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. Not all natural oligosaccharides occur as components of glycoproteins or glycolipids. Some, such as the raffinose series, occur as storage or transport carbohydrates in plants. Others, such as maltodextrins or cellodextrins, result from the microbial breakdown of larger polysaccharides such as starch or cellulose.

In biology, glycosylation is the process by which a carbohydrate is covalently attached to an organic molecule, creating structures such as glycoproteins and glycolipids.

N-Linked glycosylation involves oligosaccharide attachment to asparagine via a beta linkage to the amine nitrogen of the side chain. The process of N-linked glycosylation occurs cotranslationally, or concurrently while the proteins are being translated. Since it is added cotranslationally, it is believed that N-linked glycosylation helps determine the folding of polypeptides due to the hydrophilic nature of sugars. All N-linked oligosaccharides are pentasaccharides: five monosaccharides long.^{[citation needed]}

In N-glycosylation for eukaryotes, the oligosaccharide substrate is assembled right at the membrane of the endoplasmatic reticulum. For prokaryotes, this process occurs at the plasma membrane. In both cases, the acceptor substrate is an asparagine residue. The asparagine residue linked to an N-linked oligosaccharide usually occurs in the sequence Asn-X-Ser/Thr, where X can be any amino acid except for proline, although it is rare to see Asp, Glu, Leu, or Trp in this position.^{[citation needed]}

Oligosaccharides that participate in O-linked glycosylation are attached to threonine or serine on the hydroxyl group of the side chain. O-linked glycosylation occurs in the Golgi apparatus, where monosaccharide units are added to a complete polypeptide chain. Cell surface proteins and extracellular proteins are O-glycosylated. Glycosylation sites in O-linked oligosaccharides are determined by the secondary and tertiary structures of the polypeptide, which dictate where glycosyltransferases will add sugars.^{[citation needed]}

Glycoproteins and glycolipids are by definition covalently bonded to carbohydrates. They are very abundant on the surface of the cell, and their interactions contribute to the overall stability of the cell.^{[citation needed]}

Glycoproteins have distinct Oligosaccharide structures which have significant effects on many of their properties, affecting critical functions such as antigenicity, solubility, and resistance to proteases. Glycoproteins are relevant as cell-surface receptors, cell-adhesion molecules, immunoglobulins, and tumor antigens.