We report an optical parametric oscillator (OPO) intra-cavity pumped by an Yb-doped fiber laser. In comparison to an intra-cavity OPO based on a solid-state laser gain medium, the benefits of using a fiber-based scheme include a superior beam quality of the generated mid-infrared idler light at high power levels, a more efficient process of nonlinear frequency conversion, and the prospect of scaling to higher power. In a preliminary experiment, we obtained a slope efficiency of the down-converted power of as high as 66% with respect to the absorbed laser diode power. To the best of our knowledge, this is the first demonstration of an intra-cavity OPO based on a fiber laser.