Usage: 3 forms in 3 verses
Form Translated As     In Verse(s)
οιώνισμά is as an omen LXX.1Sam.15.23
οιωνισμάτων of your omens LXX.Jer.27.9
οιωνίσματα omens LXX.Jer.14.14

Liddell-Scott-Jones

οἰώνισμα (oiōnisma) an omen G:N-N

οἰώνισμα-ισμα, ατος, τό,

omen from the flight or cries of birds, οἰωνίσματ᾽ οἰωνῶν μαθών [LXX+5th c.BC+]; οἰ. τῆς ὑγιείας, = Salutis augurium, [Refs]; a portent, monster, [Refs 2nd c.AD+]

LSJ Scripture Refs: LXX.1Kgs.15.23LXX.Jer.14.14