The detection of chemical inhomogeities in the Galactic disk requires an oustanding precision in the abundance measurements and a thorough estimation of the uncertainties. So far, studies in alpha-elements in disk stars either do not reach the required precision, or comprise too small samples in the solar neighborhood. Thanks to the Apache Point Galactic Evolution Experiment (APOGEE), we have for the first time a large spectroscopic sample of about 100.000 disk stars, with data homogeneously obtained, reduced, and analyzed. Taking advantage of such database, we examine the distribution of oxygen-to-iron abundance ratio in stars across the Galactic disk. These data reveal that the square root of the star-to-star cosmic variance in the [O/Fe] at a given metallicity is about 0.03 to 0.04 dex in both the thin- and thick-disk populations. Measuring the spread in [O/Fe] and other abundance ratios can provide strong constraints for models of Galactic chemical evolution. In this talk we will describe how we arrived at this result, the calculation of uncertainties, and implications regarding the chemical evolution of the Galaxy.