From Proto-Indo-European *h₃ep-ni- (“working”), from the verbal root *h₃ep- (“to work > to possess”). Related to ops and opus. It could also reflect the base Proto-Indo-European *h₁op- (“to work, to take”) (c.f. optō), which de Vaan gives a slight preference for semantic reasons.
omnis m and f, omne n; third declension
- (singular) every
- (plural) all
Derived terms 
See also 
- Michiel de Vaan, Etymological Dictionary of Latin and the other Italic Languages, Brill, 2008, page 428