A phase diagram-assisted powder processing approach is shown to produce low-oxygen (0.06 wt%O) ZrB2 ceramics using minimal B4C additions (0.25 wt%) and spark plasma sintering. Scanning electron microscopy and scanning transmission electron microscopy with elemental spectroscopy are used to identify "trash collector" oxides. These "trash collector" oxides are composed of manufacturer metal powder impurities that form discreet oxide particles due to the absence of standard Zr-B oxides found in high oxygen samples. A preliminary Zr-B-C-O quaternary thermodynamic database developed as a part of this work was used to calculate the ZrO2-B4C pseudobinary phase diagram and ZrB2-ZrO2-B4C pseudoternary phase diagrams. We use the calculated equilibrium phase diagrams to characterize the oxide impurities and show the direct reaction path that allows for the formation of ZrB2 with an oxygen content of 0.06 wt%, fine grains (3.3 mu m) and superior mechanical properties (flexural strength of 660 MPa).