Oxygen concentrations were consistently below our detection limit of 90 nM for a distance of > 2000 km in the oxygen minimum zone (OMZ) along the coasts of Chile and Peru. In most cases, anammox and denitrification were only detected when in situ oxygen concentrations were below detection, with denitrification being most sensitive to oxygen. During experiments in the OMZ off Chile, we manipulated and monitored oxygen concentrations and quantified anammox and denitrification and the expression of functional N-cycle genes. Denitrification and anammox followed Michaelis-Menten type inhibition kinetics with 50% inhibition at 250 nM and 1480 nM oxygen for denitrification and anammox, respectively. Gene expression analysis indicated that a large number of different microorganisms participated in the dissimilatory N-pathways, that different groups participated in each step, and that these reacted differently to oxygen. When normalized to a housekeeping gene (rpoB), the expression of 4 out of 9 N-cycle-genes changed with increasing oxygen concentration: The expression of ammonium monooxygenase (amoC) was stimulated, whereas expression of nitrite reductase (nirS), nitric oxide reductase (norB), and nitrous oxide reductase (nosZ) were inhibited by oxygen