word-forming element meaning "toward; against; before; near; across; down," also used as an intensive, from Latin ob (prep.) "in the direction of, in front of, before; toward, to, at, upon, about; in the way of; with regard to, because of," from PIE root *epi, also *opi "near, against" (see epi-).

assimilated form of ob- before -c-.

before vowels oct-, word-forming element meaning "eight," from Greek okta-, okt-, from PIE *okto(u) "eight" (see eight). The variant form octo- often appears in words taken from Latin, but the Greek form is said to be the more common in English.

also oino-, word-forming element meaning "pertaining to wine," from Greek oinos "wine" (see wine (n.)).

assimilated form of ob- before -f-.

word-forming element meaning "oil" or "oleic," from Latin oleum (see oil (n.)).

before vowels olig-, word-forming element meaning "few, the few," from Greek oligos "few, scanty, small, little," in plural, "the few;" a word of uncertain origin.

word-forming element meaning "all," from Latin omni-, combining form of omnis "all, every, the whole, of every kind," a word of unknown origin, perhaps literally "abundant," from *op-ni-, from PIE root *op- "to work, produce in abundance."

before vowels omphal-, word-forming element meaning "navel," from Greek omphalos (see omphalos).

before vowels oneir-, word-forming element meaning "of or pertaining to a dream or dreams," from Greek oneiros "a dream," a word of uncertain origin, perhaps related to Armenian anurj, Albanian (Gheg) âdërrë.