We show that optical parametric generation in a nonlinear crystal with a large group velocity mismatch between the pump and nearly-degenerate signal and idler is analogous to laser amplification in the medium with a gain recovery time comparable to the walk-off time. Based on this conclusion we propose to combine an OPO with a nonlinear saturable absorber or Kerr lens to generate directly high peak power sub-picosecond pulses using pump pulses ranging from tens of picoseconds to quasi-CW. Our analytical model predicts better than 80% photon conversion efficiency and pulse lengths that are of the order of a few hundred femtoseconds. Numerical simulations confirm our predictions and show that repetitive passive mode locking is feasible with a quasi-CW pump.