Catalyzes the oxidation of hydroxylamine to nitrite. The electrons released in the reaction are partitioned to ammonium monooxygenase and to the respiratory chain. The immediate acceptor of electrons from HAO is cytochrome c-554. 4 Fe(III)-[cytochrome c] + H2O + hydroxylamine = 4 Fe(II)-[cytochrome c] + 5 H(+) + nitrite 3 Fe(III)-[cytochrome c] + hydroxylamine = 3 Fe(II)-[cytochrome c] + 3 H(+) + nitric oxide Binds 8 heme c groups per subunit. One specific heme c group is called heme P460. Homotrimer. Binds 8 heme groups per subunit.