Although it is impossible to know the subjective experience of another animal with certainty, the balance of the evidence suggests that most invertebrates do not feel pain. The evidence is most robust for insects, and, for these animals, the consensus is that they do not feel pain⁶ https://sencanada.ca/content/sen/committee/372/lega/witn/shelly-e.htm#:~:text=Conclusions,do%20not%20feel%20pain6.

Ganglia is the precursor for a brain but causes a primitive reaction to stimuli such as conditional changes (being pulled from the water and closing the shell)

Unlike cows, chickens and fish, oysters dont produce a waste product that pollutes. No methane makes the carbon footprint smaller. They also don't require traditional feeds which contribute to the fertilizers in the environment, water consumption of fields and land clearing to establish farms

Oysters are farmed where habitat impact is zero. Any impact to to the substrate below is positive. Studies have shown that oysters farms provide and promote new habitat for animals and sea grasses. No additional crop growth to feed oysters and no fertilizers to grow crops

If you ate oysters you could cut out the majority of your supplements needed to stay on a vegan diet