The Okipa ceremony of the Mandan Indians has numerous characteristics that differentiate it from the general model of the Sun Dance. This paper analyzes its founding myths and rituals and shows how the theme of the relationship with the buffaloes constitutes the nucleus at the base of various symbolic economic, anthropological, and cosmic codes. With the celebration of the Okipa it was possible to open the hunting season and refound and restore the values connected with this activity; the human group was renewed through initiatory elements and its identity was reshaped; the boundaries of the cosmos were redefined, evoking and overcoming potential catastrophes. In this way the ritual acted on the mutable, producing the stability of the cosmos for a whole year and annually rebuilt the presence of the Mandan in the world.