This root form (lemma) is used in 4 different forms in the NT: ὀγδόῃ (DFS), ὄγδοον (AMS), ὄγδοος (NMS), ὄγδοός (NMS).
It is glossed in 3 different ways: ‘an eighth’, ‘the eighth’, ‘eighth’.
OET LUKE 1:59 Greek word=ὀγδόῃ (ogdoaʸ) Morphology=DFS English gloss=‘eighth’ SR GNT LUKE 1:59 word 8
OET ACTS 7:8 Greek word=ὀγδόῃ (ogdoaʸ) Morphology=DFS English gloss=‘eighth’ SR GNT ACTS 7:8 word 18
OET 2PET 2:5 Greek word=ὄγδοον (ogdoon) Morphology=AMS English gloss=‘the eighth’ SR GNT 2PET 2:5 word 7
OET REV 17:11 Greek word=ὄγδοός (ogdoʼos) Morphology=NMS English gloss=‘an eighth’ SR GNT REV 17:11 word 13
OET REV 21:20 Greek word=ὄγδοος (ogdoos) Morphology=NMS English gloss=‘eighth’ SR GNT REV 21:20 word 13