/ Dictionary / Index O Oligocene: Meaning and Definition of Find definitions for: Ol•i•go•cene Pronunciation: (ol'i-gō-sēn"), [key] — Geol. Geol. —adj. noting or pertaining to an epoch of the Tertiary Period, occurring from 40 to 25 million years ago. See table under —n. the Oligocene Epoch or Series. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: oligocene (Thesaurus) oligo- oligochaete Related Content Daily Word Quiz: eccentric Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language