Translation

oisutā means "Oyster" in English.
It's a katakanization, a loan-word.

oisutā is normally written with katakana, as オイスター.
oisutā written with hiragana is おいすたー.
oisutā does not have kanji.
Romaji o i su taa
Katakana
Hiragana

Orthography Notes

Further Reading

Wikipedia: 牡蛎 (Oyster)