The headword, a poetic word typically used in oblique cases of the singular for an emotional vocalization, is a feminine noun in the accusative singular. See generally LSJ s.v. o)/y, h(, o)po/s, and cf. omicron 1068, delta 891, omicron 471, and omega 290 (end). The headword is first attested at, and here extracted from, Homer, Iliad 2.182 (web address 1), where Odysseus heeds the voice of Athena. A scholion (= D scholia) to that passage provides the same gloss (see next note).
[1] The gloss is the same form as the headword. (See generally LSJ s.v. fwnh/.) The headword is identically glossed in the Synagoge and in Photius' Lexicon (omicron389 Theodoridis). Besides the Homeric scholia (see preceding note), it is similarly glossed by Orion [Author, Myth], Etymologicum 120.11; Hesychius omicron965 s.v. o)/pa; Etymologicum Magnum 627.10-2 (Kallierges); Etymologicum Gudianum 431.21-30 (Sturz) and see De Stefani, p. 64; and Choeroboscus, Epimerismi in Psalmos 57.19-25 (Gaisford). Lexica Segueriana 319.1 has the same gloss, but in evident error transmits the headword with a rough breathing: o(/pa.