We present a single-resonant optical parametric oscillator pumped far below threshold as a source for narrow-band single photons. Spontaneous generation of single photons via parametric downconversion is modified and follows the cavity transfer function. Cross-correlation measurements between signal and idler beams show a cavity bandwidth of 62 MHz. As the main improvement to prior realizations, our cavity is locked to the pump beam via the Hänsch-Couillaud method and therefore allows the continuous generation of heralded single photons with long-term stability.