The linewidth of the ${\mathrm{BaGa}}_4{\mathrm{Se}}_7$ (BGSe) optical parametric oscillator (OPO) was narrowed for the first time, to the best of our knowledge, by inserting a Fabry–Perot (FP) etalon into an L-shaped cavity. When a 15 mm long BGSe (56.3°, 0°) was pumped by a 1064 nm laser, the peak wavelength was $\sim 3529\mathrm{nm}$, and the linewidth was 4.53 nm ($3.64{\mathrm{cm}}^{-1}$) under type I phase matching. After inserting a 350 µm thick FP etalon, the linewidth was decreased to 1.27–2.05 nm. When the tilt angle of the etalon was 2.34°, the linewidth was 2.05 nm ($1.65{\mathrm{cm}}^{-1}$), and the peak wavelength was still $\sim 3529\mathrm{nm}$. When the tilt angle of the etalon was 3.90°, the peak wavelength was 3534.9 nm, and the linewidth was 1.27 nm ($1.02{\mathrm{cm}}^{-1}$), which was the narrowest linewidth of a BGSe OPO, to the best of our knowledge. The beam quality was also improved after inserting the FP etalon.