Stellar evolutionary tracks, isochrones, and ZAHB loci have been constructed for [Fe/H]=-2.27 and four mixtures of the heavy elements to investigate how predicted C-M diagrams and age versus turnoff-luminosity relations are affected by varying the assumed abundance of oxygen (with and without enhancements in the other α-elements). These four mixes correspond to: (i) [m/H]=0.0; (ii) [m/H]=0.0, but [O/Fe]=0.3; (iii) [α/Fe]=0.3, all other metals in scaled-solar proportions; and (iv) [α/Fe]=0.3, but [O/Fe]=1.0. Model atmospheres and synthetic color-T e f f relations were computed for each of these cases. In agreement with previous work, each increase of 0.3 dex in [O/Fe] implies a 1 Gyr age reduction at a fixed turnoff luminosity. Neither the location of the main sequence nor the position of the giant branch are affected significantly by a change in the O abundance, though the distribution of stellar masses on the horizontal branch is predicted to be quite sensitive to the adopted [O/Fe] value.