Catalyzes the conversion of heme O to heme A by two successive hydroxylations of the methyl group at C8. The first hydroxylation forms heme I, the second hydroxylation results in an unstable dihydroxymethyl group, which spontaneously dehydrates, resulting in the formyl group of heme A. 2 A + Fe(II)-heme o + H2O = 2 AH2 + Fe(II)-heme a Porphyrin-containing compound metabolism; heme A biosynthesis; heme A from heme O: step 1/1. Interacts with CtaB. The N-half (TM1-TM4) and C-half (TM5-TM8) domains are connected by an intracellular loop. Each domain is formed from four-helix bundles and they align in a pseudo twofold symmetry manner. The N-half domain is the substrate-heme binding domain and the C-half domain is the cofactor heme binding domain. Mutations alter both cytochrome aa3 formation and sporulation. Belongs to the COX15/CtaA family. Type 1 subfamily.