The theoretical analysis and experimental results of the wavelength tunability of a tandem optical parametric oscillator (TOPO) based on a single nonlinear crystal are presented. TOPO is a configuration wherein the signal laser is used as a pump laser to generate secondary optical parametric oscillator (OPO). The cascaded parametric interactions are achieved synchronously in a single-grating-period MgO doped periodically poled lithium niobate (PPMgOLN). Tunable multiple-wavelength mid-infrared (mid-IR) lasers are obtained by changing the temperature of the crystal. When the PPMgOLN crystal with a grating period of 31.2 μm is operated at 148 oC, the dual OPOs generate an identical mid-IR laser of 2.83 \mum. The secondary OPO transforms into an optical parametric amplifier, in which different frequency mixing from the signal laser results in the amplification of the idler laser in the first OPO. TOPO is a useful configuration for multiple laser output, broad tuning range, and high-efficiency mid-IR lasers.