The tragic poets never have the ω-forms; the poets of the Old Comedy seldom; those of the New Comedy often have the ω-forms.—Plato usually has -νυα_σι. Hom. has ζεύγνυον (and ζεύγνυσαν, ὤρνυον, ὤμνυε, ὀμνυέτω, etc.). Hdt. usually keeps the μι-forms, but has some ω-forms in 2, 3 sing. 3 pl. present indic. and part., and 1 sing., 3 pl. imperfect. Doric usually has the ω-forms; Aeolic has ζεύγνυ_, and ὄμνυ_ν infin.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].