Catalyzes the four-electron reduction of molecular oxygen to water. a ubiquinone + 5 H(+)(in) + NADH = a ubiquinol + 4 H(+)(out) + NAD(+) Binds 1 FAD per subunit. The active site is the redox-active Cys-44 oxidized to Cys-SOH. The oxidized form is stabilized by a hydrogen bond formation with His-11 (By similarity). Belongs to the class-III pyridine nucleotide-disulfide oxidoreductase family.