This paper presents the theoretical study of a monolithically integrated NEMS/CMOS oscillator with electrostatic actuation and piezoresistive detection. A feedback circuit based on a single active transistor is implemented. The proposed architecture is so compact that it can be implemented with ease in a sensor array application for example. A brief description of the NEMS resonator is given and the conditions for oscillation build-up are stated. We show how the co-integration allows the use of a very simple sustaining electronic circuit. This paper focuses on the determination of the system steady-state and more specifically the amplitude of the mechanical displacement and electronic output voltage. We show that the intrinsic nonlinearities of the electronic circuit can be used to achieve stable oscillation. The proposed oscillator exhibits a large output signal while keeping small mechanical amplitude compared to the electrostatic gap.