We report a compact, efficient, high-energy, and high-repetition-rate mid-IR picosecond optical parametric oscillator (OPO) based on the new nonlinear material ${\mathrm{CdSiP}}_2$ (CSP). The OPO is synchronously pumped by a master oscillator power amplifier system at $1064.1\mathrm{nm}$ , providing $1\mathrm{\mu s}$ long macropulses constituting $8.6\mathrm{ps}$ micropulses at $450\mathrm{MHz}$ , and it can be tuned over $486\mathrm{nm}$ across $6091–6577\mathrm{nm}$ , covering the technologically important wavelength range for surgical applications. Using a compact ( $\sim 30\mathrm{cm}$ ) cavity and improved, high-quality nonlinear crystal, idler macropulse energy as high as $1.5\mathrm{mJ}$ has been obtained at $6275\mathrm{nm}$ at a photon conversion efficiency of 29.5%, with $>1.2\mathrm{mJ}$ over more than 68% of the tuning range, for an input macropulse energy of $30\mathrm{mJ}$ . Both the signal and idler beams are recorded to have good beam quality with a Gaussian spatial profile, and the extracted signal pulses are measured to have durations of $10.6\mathrm{ps}$ . Further, from the experimentally measured transmission data at $1064\mathrm{nm}$ , we have estimated the two-photon absorption coefficient of CSP to be $\beta =2.4\mathrm{cm}/\mathrm{GW}$ , with a corresponding energy bandgap, $E_g=2.08\mathrm{eV}$ .