A key part of the respiratory and photosynthetic pathways is the bc1 protein complex embedded in the innermembrane of mitochondria and the plasma membrane of photosynthetic bacteria. The protein complex pumps protons across the membrane to maintain an electrostatic potential, which is in turn used to drive ATP synthesis. This molecular machinery however, is suspected to be a source of superoxide, which is toxic to the cell, even in minuscular quantities, and believed to be a factor in aging. Through molecular dynamics simulations, we investigate here the migration of molecular oxygen in the bc1 complex in order to identify possible reaction sites that could lead to superoxide formation. It is found, in particular, that oxygen penetrates spontaneously the Qo binding site of the bc1 complex in the presence of an intermediate semiquinone radical, thus making the Qo-site a strong candidate for being a center of superoxide production.