The headword, attested principally in classical Attic oratory, is a neuter noun in the nominative/accusative singular; see generally LSJ s.v., citing instances which include Demosthenes 21.99.
cf. more generally omicron 1013, omicron 1015, omicron 1016, omicron 1017, omicron 1018, and omicron 1019.
[In her critical apparatus Adler notes that ms A omitted this entry.]
[1] The gloss is of the same grammatical form as the headword, which is identically glossed in Hesychius omicron1973, the Synagoge, and Photius' Lexicon (omicron712 Theodoridis); cf. also Lexica Segueriana 323.2 s.v. o)fqalmia/sai. Adler also cites Boysen's Lexicon Seguerianum and the Ambrosian Lexicon (670).