Catalyzes the four-electron reduction of molecular oxygen to water. In fact, functions as the terminal component of an NADH oxidase (NADH:O(2) oxidoreductase) when using NADH:rubredoxin oxidoreductase (NROR) and rubredoxin (Rd) as electron transport intermediaries between NADH and FDP. Is thus able to reductively scavenge intracellular dioxygen and is part of an oxidative stress defense system in C.acetobutylicum, an obligate anaerobic bacterium. 2 H(+) + 2 NADH + O2 = 2 H2O + 2 NAD(+) Binds 1 FMN per subunit. Binds 2 iron ions per subunit. Homodimer. Up-regulated upon exposure to O(2) (at mRNA and protein levels). Repressed by PerR. Consists of an N-terminal non-heme diiron domain and a C-terminal flavodoxin-like domain. In the N-terminal section; belongs to the zinc metallo-hydrolase group 3 family.