High-Mg Kroenke-type basalts containing 9–11 wt% MgO from the Ontong Java Plateau (OJP) include spinels. These spinels have nearly identical Cr/(Cr + Al) ratios (0.45–0.54), and are generally hosted by olivine phenocrysts with relatively primitive compositions, with Fo contents [100 × Mg/(Mg + Fe²⁺) in atomic ratios] as high as 88. This implies that the primary OJP magmas were in equilibrium with refractory peridotites (i.e., harzburgites) and were homogenized by large-scale melting and magmatic evolutionary processes. Oxygen geobarometry indicates that the OJP primary magmas record uniform oxygen fugacity (fO₂) conditions and are slightly more oxidized (0.6 log units) than normal mid-ocean ridge basalts, but record less-oxidized fO₂ conditions than ocean island basalts. These data are consistent with previous studies that suggest the OJP primary magmas were generated by large-scale and extensive melting of a mantle source region under relatively oxidized conditions.