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. Belongs to the COX15/CtaA family. Type 1 subfamily.