The oyster keeps its treasure inside a rough shell, so it is read as worth that has to be opened up, or a closed-off feeling guarding something precious.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.