Periplasmic nitrate reductase catalytic subunit (NapA) belongs to the DMSO reductase (DMSOR) family of molybdenum cofactor (Moco) containing enzymes and catalyzes nitrate reduction to nitrite. Interestingly, NapA from Campylobacter jejuni can catalyze the reduction of a wide array of substrates such as DMSO, TMAO, arsenate, perchlorate, chlorate, and iodate. The wild-type enzyme can also oxidize nitrite to nitrate. The mechanism of nitrate reduction is generally thought to involve the oxygen atom transfer (OAT) process. Using isotope labeling, mass spectrometry, and NMR experiments, we provide evidence of OAT reaction nitrate reduction and nitrite oxidation in this enzyme. We also discuss the mechanism of nitrate reduction that is consistent with the active site structure.