In this paper, we investigate the optimal operating conditions of electrostatically-actuated MEMS oscillators in terms of frequency stability. Through a simple model, the influence of the fluctuation of several design parameters (e.g. feedback phase, feedback amplitude, bias voltage) on oscillator performance is studied. It is shown that, in the nonlinear regime, the excitation waveform (e.g. square, sinusoid, impulsive, etc.) plays a critical role on the long-term frequency stability of electrostatic MEMS oscillators, as opposed to the well-known but idealized case of a Duffing oscillator.