Usage: 2 forms in 10 verses
Form Translated As     In Verse(s)
οκτωκαιδεκάτω eighteenth LXX.1Kgs.15.1LXX.2Kgs.3.1LXX.2Kgs.22.3LXX.2Kgs.23.23LXX.2Chr.13.1LXX.2Chr.34.8LXX.2Chr.35.19
οκτωκαιδέκατοσ eighteenth LXX.1Chr.24.15LXX.1Chr.25.25LXX.Jer.32.1

Liddell-Scott-Jones

ὀκτωκαιδέκατος (oktōkaidekatos) eighteenth G:A

ὀκτωκαιδέκατος-ος, η, ον,

eighteenth, -δεκάτη (i.e. ἡμέρᾳ) on the eighteenth day, [Refs 8th c.BC+]; Boeotian dialect ὀκτοκηδέκατος [Refs 3rd c.BC+]