This paper focuses on the hardware and software co-design of a self-sustaining and energy efficient smart ring that includes a pulse oximeter to monitor blood oxygenation continuously and perpetually. The papers exploit the combination of energy harvesting and energy efficient onboard processing with the goal to have a neutral energy ring even using a small and flexible solar panel that works in indoor light condition. Moreover, the paper evaluates the energy efficiency of the onboard processing over the wireless transmission in the target application.