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We examine theoretically under what conditions mirrorless optical parametric oscillation can be obtained in PPLN and PPLT. We take into account the damage threshold, idler absorption and domain width fluctuations, as well as competing nonlinear processes that may deplete the pump before oscillation occurs. We also solve numerically the coupled wave equations to take into account the buildup from quantum fluctuations to a detectable level. We found that there is a pulsewidth that minimizes the required pump energy to observe mirrorless oscillation. For a perfect 2-cm-long PPLN sample, a 700-ps, 1.064-µm pump with a 50-µm radius and a counter-propagating 4.5-µm idler, fifth-order quasi-phase-matched oscillation can be obtained with less than 70 µJ. For PPLT, the requirements are higher (200 µJ for a perfect 2.5-cm PPLT sample). Mirrorless oscillation in bulk PPLN and PPLT requires almost …