Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ōgyges, is, Ōgygus, i, and Ōgy-gĭus, ĭi, m., = Ὠγύγης, Ὤγυγος, Ὠγύγιος, the mythic founder and king of Thebes, in Bœtia, in whose reign a great deluge is said to have occurred: oppidum Thebae, quod rex Ogyges aedificarit, Varr. R. R. 2, 1, 2: ante cataclysmon Ogygi, id. ib. § 3: hoc factum Ogyge rege dicebant, Varr. Fragm. ap. Aug. Civ. Dei, 21, 8.
Form Ogygus, Paul. ex Fest. p. 179 Müll.
Form Ogygius, Aug. Civ. Dei, 18, 8.
Hence,
Ōgygĭus, a, um, v. Ogyges, B.