Atomic oxygen abundance relative to hydrogen nuclei as a function of velocity. The mean value of the 120 individual channels within the −120 and 40 km s⁻¹ velocity range is 2.51 × 10⁻⁴, and the dispersion of the individual channels is 0.65 × 10⁻⁴. The horizontal dotted lines mark the mean value and ±1σ dispersion computed from the ensemble of individual measurements. The black vertical error bars mark ±1σ uncertainties in the individual channels, as described in the text. Color bars mark velocity ranges corresponding to the 3 kpc, 5 kpc, Sagittarius, and Scutum arms (red, magenta, blue, and yellow, respectively). Velocities corresponding to the Galactic center gas are marked in green and those of the local gas in gray. Channels with saturated absorption are masked. An electronic table with the data used for this figure is available at the CDS.