We derive the mathematical condition for the existence in an optical parametric oscillator of an optimum tuning point (OTP), the point of maximum change of signal or idler frequency per unit perturbation by some applied influence. Computer calculations show that such a point exists in LiNbO3, KDP, ADP, LiIO3, and AgGaS2. A graphical interpretation of the OTP is presented. The pump frequency and crystal orientation at the OTP as well as expected tuning ranges for electric field tuning are given for the above five crystals.