Oyster comes through Middle English and Anglo-French from Latin ostrea and Greek óstreon.
Anglo-French also had oistre, so ostre and oistre are not being ranked as successive changes within French. The deeper Greek ancestry is uncertain.
Sources
- Merriam-Webster: oyster
Word History gives the Greek, Latin, Anglo-French and Middle English variants; its note qualifies the deeper ancestry.
- Anglo-Norman Dictionary · oistre
Meanings of the older forms on the word path.
- perseus.tufts.edu · text
Meanings of the older forms on the word path.
- Online Etymology Dictionary · oyster
Meanings of the older forms on the word path.