A highly efficient mirrorless optical parametric oscillator (MOPO), pumped by narrowband nanosecond pulses at 1064 nm, is demonstrated. The MOPO is based on quasi-phase-matched parametric interaction of counter-propagating photons in 1-mm-thick periodically poled Rb-doped KTiOPO4 crystal with a period of 755 nm. It generates a co-propagating signal at 1740 nm and a counter-propagating idler at 2741 nm, achieving mJ-level output with a total signal-and-idler conversion efficiency of 47%. Both generated waves present narrow spectral bandwidths, thanks to the unique properties of the counter-propagating nonlinear interaction. The high conversion efficiency, inherently narrow spectral width, and simplicity of the optical setup make the MOPO an attractive alternative to conventional co-propagating optical parametric oscillators.