The evolution of the South Asian monsoon (SAM) is not well-constrained prior to the Pleistocene, primarily due to a lack of recovered marine sediment archives. This study presents geochemical data from Site NGHP-01-01A in the eastern Arabian Sea from the Oligocene to Early Miocene (~32–20 Ma) to reconstruct the oceanographic evolution of this region through this interval. Analyses consist of planktic and benthic foraminiferal oxygen and carbon isotopes, and planktic foraminiferal trace elements (Mg/Ca and Mn/Ca), as well as seawater d18O (d18Osw) and Mg/Ca-derived temperatures. Along with XRF-derived elemental analysis, these reveal the Oligocene–Miocene transition (~23.7–22.7 Ma) to be a key initiation/intensification of the proto-SAM system, with the development of an intense oxygen minimum zone and monsoonal atmospheric circulation.