—New petrological, isotope-geochronological, and geophysical data reveal two phases of magmatism in the Oortsog mafic–ultramafic intrusion in Western Mongolia. The intrusion consists of (i) rhythmically layered peridotite gabbro (278.7 ± 2.5 Ma) and (ii) poorly differentiated biotite-containing amphibole–olivine gabbro and gabbronorite (272 ± 2 Ma). The inverse upward succession from leucocratic to melanocratic lithologies within each rhythm indicates that the intrusion was tectonically overturned. The earlier rocks (phase 1) have lower contents of major oxides (Na2O + K2O, TiO2, and P2O5) than the later ones (phase 2) and show different patterns of incompatible elements. The intrusive rocks of phases 1 and 2 were derived from depleted (positive εNd) and enriched (negative εNd) mantle sources, respectively. The calculated parental melts of both phases belong to picritic (Mg-rich) basaltic magma. More insight into the intrusion structure was gained by mapping magnetic anomalies.