Olivine-phyric shergottites (basaltic cumulates) are primitive martian meteorites with members in three defined geochemical reservoirs (enriched, intermediate, and depleted) based on LREE enrichment, and are ideal candidates for investigating detailed olivine chemistry with respect to shergottite petrogenesis. Olivine serves as a physiochemical trace, preserving information about mantle source lithology and formation pressure-temperature conditions. High precision electron microprobe analyses of olivine megacrysts in a suite of 13 olivine-phyric shergottites reveal enriched shergottites are distinguishable from the depleted/intermediate groups based on nickel content at a given forsterite content. The same distinctions cannot be made based on calcium or manganese content in olivine. Olivine chemistry, bulk rock indices, and olivine-spinel co-crystallization temperatures do indicate variations in source lithology. However, Ni partitioning in olivine is affected by pressure, and high-pressure formation of the enriched shergottites in the martian mantle is consistent with previous experiments and the Ni content observed in this study.