oisuta or kaki
 

Hi I am new to this but hope that someone is kind enough to help me out.

Could someone explain whether there is or not a difference between using oisuta or kaki as a translation for 'oyster'? Do they both just mean 'oyster'? I think i noticed that 'oisuta' is also used when it is about combinations like 'oyster sauce', ... but does it make perfect sense on its own?

「オイスター」 is NOT used to refer to oysters. The word we use is always 「牡蠣（かき）」 to refer to an oyster, either with or without the shells.　　

Words like 「オイスターソース」 is exceptional because it is treated almost like a proper noun. It is also called 「かきソース」.

If a certain restaurant in Japan happens to calls a certain oyster dish 「ＸＸＸオイスター」, then you just call it the same way. You do not "translate" those kinds of terms.
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For those interested in pitch accents:
Pitch-accented syllables in red
かき = an oyster
かき = a persimmon

Just so you won't ask for oysters at a produce market. :)