The headword is a masculine noun in the accusative singular; see LSJ s.v. o(lko/s, and cf. omicron 176, and omicron 180. (There is also an adjective, o(lko/s, -h/, -o/n, drawing to oneself, attractive; cf. omicron 177, omicron 178, and omicron 179.) The headword is perhaps extracted from the quotation given, though the other lexica [next note] do not have it, and there are numerous other possibilities.
[1] The glossing substantives are all in the accusative singular, the first three masculine, the last feminine. The headword is identically glossed in the Synagoge, Photius' Lexicon (omicron230 Theodoridis), and Lexica Segueriana 316.19; cf. Hesychius omicron587-588.
[2] Menander Protector, De legationibus Romanorum ad gentes 5.19-20 (= fr. 15 FHG, 4.220); cf. upsilon 69.