This work presents an ultra-low power oscillator designed to target different contexts, such as crystal-assisted time keeping, reference oscillator to optimize the always on domain of a microcontroller or wake-up timer. This oscillator enables ultra-low power operation in 0.18 mu m CMOS technology; the core oscillator consumes 2.5 nW at room temperature, with a temperature stability of 14 ppm/degrees C [-40 degrees C -60 degrees C] and 0.07 %/V supply sensitivity.