[en] An optical parametric oscillator with a periodically poled KTiOPO₄ (KTP) as a gain medium pumped by a pulsed Nd:YAG green laser with a maximum pulse repetition frequency of 100 Hz is demonstrated. Output energy of 5.8 mJ at the pulse repetition frequency of 100 Hz with the conversion efficiency of about 40% is obtained by a double-pass pumping scheme. Threshold pumping energy is 1.6 mJ with a peak power as low as 18 MW/cm². At a pulse repetition frequency between 10 Hz and 100 Hz, the conversion efficiency is almost constant and undesirable thermal effects are not observed. (author)