Cavity ring-down detection techniques can sensitively determine frequency-dependent absorption cross-sections of gasses. However, so-called line-width problems and amplified spontaneous emission of the laser light source lowers the technique’s quantitative accuracy. Using phase-shift cavity ring-down spectroscopy (PSCRD), we measured absolute line intensities of the spin-forbidden transitions in the band of molecular oxygen. Our results were within 4% of values obtained from the HITRAN database, demonstrating the accuracy of PSCRD, when corrected for amplified spontaneous emission. Its high sensitivity (2 × 10-8 cm-1), simplicity and high duty cycle make PSCRD a powerful diagnostic technique.