I am not a biologist but my understanding is that largely has to do with a lack of central nervous system. It would be like asking if a heart is aware of itself. It can autonomously react to things like low oxygen but that isn’t because those signals go anywhere that makes a decision it’s more like a chemical/biological Rube Goldberg machine. If you really want to get down to it though I don’t think we can know for certain just make educated guesses, and imo oysters are even less likely to have any form of consciousness than a lot of plants or mushrooms