An experiment enabling extremely high signal-to-noise ratios in the measurement of spectral line shapes is described. This approach, which combines high-bandwidth locking of a continuous wave probe laser and the frequency-stabilized cavity ring-down spectroscopy technique, enables long-term signal averaging and yields high-resolution spectra with a relatively wide dynamic range and low detection limit. By probing rovibronic transitions of the O band near nm, exceptionally precise measurements of absorption line shape and line position are demonstrated. A signal-to-noise ratio of and a minimum detectable absorption coefficient of is reported, which corresponds to the lowest line intensity measurable by this setup of approximately . Careful analysis of the data revealed a subtle line-shape asymmetry that could be explained by the speed dependence of the collisional shift. The demonstrated measurement precision enables the quantification of systematic line-shape deviations, which were approximately 1 part in 80 000 of the peak absorption. The influence of slowly drifting etaloning effects on the precision of the line-shape analysis is discussed. How this method can enable experiments that address a number of fundamental physical problems including the accurate optical measurement of the Boltzmann constant and tests of the symmetrization postulate is also discussed.