Translation

The Japanese word for "Oyster" is kaki 牡蠣.

Romaji ka ki
Hiragana
Katakana
Kanji 牡蠣
School Level ,

Loan-word

The word "Oyster" katakanized would be oisutā オイスター.
Romaji o i su taa
Katakana
Hiragana

Further Reading

Wikipedia: Oyster (牡蛎)