Deoxygenation is a major threat to the coastal ocean health as it impacts marine life and key biogeochemical cycles. Understanding its drivers is crucial in the thriving and highly exploited Peru upwelling system, where naturally low-oxygenated subsurface waters form the so-called oxygen minimum zone, and a slight vertical shift in its upper limit may have a huge impact. Here we investigate the long-term deoxygenation trends in the upper part of the nearshore oxygen minimum zone off Peru over the period 1970-2008. We use a unique set of dissolved oxygen in situ observations and several high resolution regional dynamical-biogeochemical coupled model simulations. The upper part of the oxygen minimum zone appears to lose oxygen over the period, particularly off Northern Peru, a trend well reproduced by the model. Model simulations attribute the deoxygenation to the slowdown of the near-equatorial eastward currents, which transport oxygen toward the Peruvian shores. The large uncertainties in the estimation of this ventilation flux and the consequences for more recent and future deoxygenation trends are discussed.