1520s, "miscellaneous collection," a humorous mock-Latin coinage from Latin omnium "of all things" (genitive plural of omnis; see omni-) + a feigned Latin form of English gather. Earlier form was omnegadrium (early 15c.), omnigatherum.
omnipresence
omnipresent
omniscience
omniscient
omnisexual
omnium gatherum
omnivore
omnivorous
omo-
omophagous
omphalic