Signal and idler wave spectra from an optical parametric oscillator were analyzed. Quasi phase matching in 5 mol.\% MgO-doped periodically poled lithium niobate was used to generate infrared radiation. The dependences of the wavelengths of the signal and idler waves on the grating period and the temperature were precisely measured. The wavelengths of the signal and the idler waves deviated considerably from the predictions of Sellmeier equation, especially near the degenerate point.\