Preheat oven to 350°. Lightly butter a large rectangular baking pan. Heat 2 tablespoons of the butter in a large skillet over medium-low heat. Sauté onions and celery in the butter until tender; remove from heat an add a small pinch of cayenne and let cool.
Combine cornbread and breadcrumbs in a large bowl; gently fold in sautéed onions, salt, pepper, and parsley. Add beaten eggs and toss more; moisten with the reserved oyster liquid until moist but not soggy. Gently stir in the oysters.
Pat the mixture into the prepared baking pan (it should make a 1-inch layer in the pan). Dot with remaining butter and bake about 45 minutes, until golden brown and set in the center.