A series of oceanic anoxic events (OAEs) occurred in the mid-Cretaceous warm period (120--80 Ma) that have been linked with high rates of organic carbon burial, warm high- and low- latitude temperatures, and sea-level changes. OAEs have been studied individually, but a causal mechanism that connects them has been lacking. We show that peaks in phosphorus accumulation in marine sediments broadly coincide with OAEs 1a, 1b, 1d, 2, and 3, and exhibit a 5--6 Myr periodicity, which for reactive-P is prominent over 100--80 Ma. Oxic-anoxic oscillations of this frequency are also found in a model of the coupled N, P, C, and O2 biogeochemical cycles. These oscillations are maintained by positive feedbacks between phosphate concentration, biological productivity, and anoxia in the global ocean and counteracting, but slower, negative feedbacks involving changes in atmospheric oxygen. An increase in phosphorus weathering rate above a critical threshold can shift the system into self-sustaining oscillation. This could have been caused by tectonic and volcanic forcing increasing atmospheric CO2 and global warmth 120--80 Ma, augmented by the rise of flowering plants around 100 Ma. With a plausible forcing scenario, we are able to reproduce the approximate timing of OAEs 1a, 1b, 1d, 2, and 3 in the model.