The early Aptian Oceanic Anoxic Event (OAE 1a) resulted from an exceptional set of interactions between the geosphere, the biosphere, and the ocean-atmosphere system. We present new Re-Os data from two sites spanning OAE 1a in the Tethys and Pacific Oceans. The patterns of variation in the seawater Os-isotope composition from both sites are very similar, and together they constrain the timing and duration of continental weathering in relation to the large-scale volcanic activity of the Ontong Java Plateau. The dominant feature through the OAE is an interval of ∼880 k.y. when the Os-isotope composition of the global ocean was exceptionally unradiogenic, implicating unambiguously the Ontong Java Plateau as the trigger and sustaining mechanism for OAE 1a. A relatively short-lived (∼100 k.y.) Os-isotope excursion to radiogenic compositions in the Tethyan record is clearly linked to an abrupt perturbation to the global carbon cycle, and is fully consistent with the Pacific record. These highly distinctive features of seawater Os in contemporaneous samples from three high-resolution sections, two of which were very remote from the Ontong Java Plateau, indicate that ocean mixing at that time was very efficient. The results suggest that OAE 1a was also related to rapid global warming and elevated rates of silicate weathering both on the continents and in the oceans.