The Collaborative International Dictionary
Oligo-
Oligo- \Ol"i*go-\ A combining form from Gr. 'oli`gos, few, little, small.
Douglas Harper's Etymology Dictionary
oligo-
before vowels olig-, word-forming element meaning "few, the few," from comb. form of Greek oligos "few, scanty, small, little," in plural, "the few;" of uncertain origin.
Wiktionary
oligo-
pre. few