The Ontong Java Plateau (OJP) is one of the Large Igneous Provinces (LIPs) in the Pacific Ocean. Ten samples of tholeiitic basalt samples from site 803 and eight samples of tholeiitic basalt from site 807 from the Ontong Java Plateau have been analyzed for major, trace and rare earth elements (REE). Major element data suggest different degrees of fractionation and different parental magmas for the two sites. Major and trace element data also indicate a high degree of melting in the source regions of these magmas. Variation diagrams for major oxides show different clustering of data points for the basalts of site 803 and 807 implying different parental magmas for the basalts exposed at these two sites. Based on their MgO values (6.74% to 4.66% for site 803, and 7.29% to 6.56% for site 807), basalts from site 803 are more differentiated than basalts from site 807. Moreover, basalts from site 807 can be further distinguished as high-TiO (sub 2) basalts and low-TiO (sub 2) basalts; and this in turn, implies that there are at least two different magmas involved in producing the basalts of site 807. The chondrite-normalized LREE composition of the OJP basalts from sites 803 and 807 are similar to Mid-Ocean Ridge basalts (MORB), but quite different than Ocean Island basalts (OIB). Chondrite-normalized heavy rare earth elements (HREE) for the OJP and OIB are similar. The data suggests that there are components of both MORB and OIB in the OJP magmas, and any model proposed for their origin must address this problem. The sequence of crystallization of oceanic basalts at low pressure (1 atm) is ol-->plag-->cpx-->Fe-oxides. An interesting feature of the basalts from both sites is the virtual absence of olivine. This implies that the magma did not differentiate at low pressures, but rather at higher pressures. In order to evaluate the consequences of high-pressure fractionation on the mineralogy and residual melt composition, Ghiorso and Sack's (1995) MELTS program was used under the constraints of isobaric conditions at different pressures. Results indicate that the actual liquid line of descent for the OJP magmas agrees with the theoretically calculated liquid line of descent between four and eight kilobars. This in turn supports the conclusion that the OJP magmas differentiated at high pressures. Using published minimum and maximum seismic velocities for plagioclase and clinopyroxene and the minerals predicted by the MELTS program, a velocity structure for the OJP has been calculated using the 4 and 8 kilobars MELTS output. Calculated P-wave velocities range from 7.10 to 7.64 km/sec for about 30% fractionation and from 6.88 to 7.32 km/sec for about 75% fractionation. These velocities correspond to a depth of 15-30 km below the surface of the ocean floor. In addition, these velocities match those measured by seismic refraction and correspond to the thick, high-velocity basal layer typically found in the larger oceanic plateaus, and the 3A layer above, respectively.