/ Dictionary / Index O onco-: Meaning and Definition of Find definitions for: onco- Pronunciation: [key] a combining form meaning “tumor,” “mass,” used in the formation of compound words: oncogenic. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. oncidium oncogene Related Content Daily Word Quiz: agitated Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language