Required for the assembly of yeast cytochrome oxidase. Involved in the biosynthesis of heme A and the initial step in this pathway, the hydroxylation of heme O, is thought to be catalyzed by a three-component mono-oxygenase consisting of COX15, ferredoxin YAH1 and ferredoxin reductase ARH1. Forms 200-350 kDa oligomeric complexes independent on heme binding. Belongs to the COX15/CtaA family.