The theoretical approach to investigate the frequency pulling phenomenon, when the optoelectronic oscillator is injected by a weak radio frequency (RF) signal, is presented. The differential equation for the phase perturbation is derived. The analytical expressions of the phase-difference after locking and lock-range are derived here. Here, we also study the influence of the injection RF signal on the beat frequency, average frequency, and actual pull-in frequency. Also, the dependency of beat frequency and average frequency on injection frequency is studied here. All the analytical expressions clearly reveal the frequency pulling phenomenon from fast-beat state through quasi-locked state to locked state. Simulation results are also given in support of the theoretical study.