Converts heme B (protoheme IX) to heme O by substitution of the vinyl group on carbon 2 of heme B porphyrin ring with a hydroxyethyl farnesyl side group. (2E,6E)-farnesyl diphosphate + H2O + heme b = diphosphate + Fe(II)-heme o Porphyrin-containing compound metabolism; heme O biosynthesis; heme O from protoheme: step 1/1. Carbon 2 of the heme B porphyrin ring is defined according to the Fischer nomenclature. In the N-terminal section; belongs to the COX15/CtaA family. In the C-terminal section; belongs to the UbiA prenyltransferase family. Protoheme IX farnesyltransferase subfamily.