Crush cracker crumbs. Layer in a  2 qt. casserole dish.  First layer, cracker crumbs, then a  layer of corn and pieces of butter,  chopped onion and drained oysters. Repeat layers until dish is full, with cracker crumbs on top.  Mix milk, salt and pepper, and oyster liquid from can of oysters and pour over all.  Dot with butter on top.  Bake at 350 degrees until lightly browned on top.