The fraction of O atoms as non-bridging O atoms (NBO) can be well approximated based on composition alone in many silicate glasses, but the NBO preference for specific network forming cations is much less well known. Using oxygen-17 (17O) NMR on low-silica calcium aluminosilicate (CAS) glasses, this study shows that Al-NBO (155 ppm) can be readily distinguished from Si-NBO (110–120 ppm), and that there is a strong preference for the latter. This study also presents a consistent equilibrium constant formulation that indicates that for thermodynamic modeling of most CAS melts with Si > Al, Al-NBO are of minor importance, although they could be significant in some models of diffusion and viscosity. Al-27 one pulse NMR and analyses of spinning side bands show that AlO5 and AlO6 species are below detection limits (<0.5%) in the low-silica (SiO2 ≤ 20 mol%) glasses of this study (NBO/T = 0.6 to 0.8). In addition, 17O MAS NMR does not detect any obvious (<2%?) Al3O triclusters; hence calculations of NBO assignments can be assigned unambiguously.