An oligomer (/əˈlɪɡəmər/ (listen)^[2]) (oligo-, "a few" + -mer, "parts") is a molecular complex of chemicals that consists of a few repeating units, in contrast to a polymer, where the number of monomers is, in principle, infinite.^[3] Dimers, trimers, and tetramers are, for instance, oligomers composed of two, three, and four monomers, respectively.

In biochemistry, an oligomer usually refers to a macromolecular complex formed by non-covalent bonding of a few macromolecules like proteins or nucleic acids. In this sense, a homo-oligomer would be formed by few identical molecules and by contrast, a hetero-oligomer would be made of more than one, different, macromolecules. Collagen is an example of a homo-oligomeric protein that is composed of three identical protein chains. The term multimer (/ˈmʌltɪmər/) (multi-, "more than one" + -mer, "part") is used with a meaning similar to that of oligomer in the context of proteins (although technical restrictions of word sense may exist).

Many oils are oligomeric, such as liquid paraffin. Plasticizers are oligomeric esters widely used to soften thermoplastics such as PVC. They may be made from monomers by linking them together, or by separation from the higher fractions of crude oil. Polybutene is an oligomeric oil used to make putty. Greek prefixes are often used to designate the number of monomer units in the oligomer, for example a tetramer being composed of four units and a hexamer of six.

In biochemistry, the term oligonucleotide – or, informally, "oligo" – is used for short, single-stranded nucleic acid fragments, such as DNA or RNA, or similar fragments of analogs of nucleic acids such as peptide nucleic acid or Morpholinos. Such oligos are used in hybridization experiments (bound to glass slides or nylon membranes), as probes for in situ hybridization or in antisense experiments such as gene knockdowns. It can also refer to a protein complex made of two or more subunits. In this case, a complex made of several different protein subunits is called a hetero-oligomer or heteromer. When only one type of protein subunit is used in the complex, it is called a homo-oligomer or homomer.

Oligomerization is a chemical process that converts monomers to macromolecular complexes through a finite degree of polymerization. The actual figure for degree of polymerization is a matter of debate, often a value between 10 and 100.

Telomerization is the process where an oligomer forms a telomer as a result of chain transfer.^[4] A telomere is a region of highly repetitive DNA at the end of a linear chromosome.

In the oil and gas industry, green oil refers to oligomers formed in all C2, C3, and C4 hydrogenation reactors of ethylene plants and other petrochemical production facilities; it is a mixture of C4 to C20 unsaturated and reactive components with about 90% aliphatic dienes and 10% of olefins plus paraffins.^[5] Different heterogeneous and homogeneous catalysts are operative in producing green oils via the oligomerization of olefins ^[6]