The problem of optical field chaos in a storage ring free-electron laser oscillator has been discussed by using a phenomenal model. From the theoritical analysis and numerical simulation, it can be found that the chaotic transition of the system via perioddoubling bifurcation still exists when there is a pulsed gain modulation, instead of an oscillated one, in the optical cavity. The existence of optical field chaos has been confirmed by calculating the corresponding value of the leading Lyapunov characteristic exponent. A further research shows that the real form of gain modulation in the optical cavity is most likely a pulsed one, not an oscillated one.