It has been demonstrated that shared cavity optical parametric oscillator (OPO) could improve power stability in comparison with that of coupled cavity OPO. By theoretically calculating the spectral transmissions induced by the etalon effect, it has been found that the shared cavity OPO had much wider transmission bandwidth than that of coupled OPO. The corresponding experimental results agreed with the theoretical analysis. Furthermore, the OPO and fundamental laser resonator have nearly the same longitudinal mode spacing in the shared cavity configuration. The above two factors can account for the improved performance of the shared OPO operation.