Catalyzes the opening of the heme ring to form the open-chain tetrapyrrole biliverdin IX with the release of iron and carbon monoxide (CO). heme b + 3 O2 + 3 reduced [NADPH--hemoprotein reductase] = biliverdin IXalpha + CO + Fe(2+) + H(+) + 3 H2O + 3 oxidized [NADPH--hemoprotein reductase] Expressed in dark and light. Part of the bphO-bphP operon (PubMed:27621284). Belongs to the heme oxygenase family.