Converts protoheme IX and farnesyl diphosphate to heme O. (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. Belongs to the UbiA prenyltransferase family.