The Collaborative International Dictionary of English v.0.48:

oligomer \o*lig"o*mer\, n. (Chem.)
   A molecule composed of a small number of linked monomer
   units; a short polymer; -- compounds called oligomers have
   less than one hundred monomer units and usually less than
   thirty. Oligomers of increasing length are called dimer,
   trimer, tetramer, pentamer, hexamer, heptamer,
   octamer, nonamer, decamer, etc. In colloquial
   laboratory jargon, they may also be referred to as
   nine-mer, ten-mer, eleven-mer, twelve-mer, etc.,
   especially for oligomers of greater than eight units.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

octamer \oc"ta*mer\ ([o^]k"t[u^]*m[~e]r), n. [Octa- + Gr. me`ros
   part.] (Chem.)
   A molecule composed of eight monomer units bound to each
   other, usually in a linear array; as, an octamer formed from
   eight nucleotides is called an octanucleotide.
   [PJC]

   Note: An example of an octapeptide might be represented using
         the standard abbreviations for the component amino
         acids, e.g.: met-ala-ser-glu-lys-ala-val-gly
         An octanucleotide might be represented using the
         standard single-letter abbreviations for the component
         mononucleotides, e.g.: ATGCATGC.
         [PJC]