FAD-dependent monooxygenase required for the aerobic hydroxylation of 2-octaprenylphenol to 2-octaprenyl-6-hydroxy-phenol, the first hydroxylation step in coenzyme Q (ubiquinone) biosynthesis. 2-all-trans-octaprenylphenol + H(+) + NADPH + O2 = 3-(all-trans-octaprenyl)benzene-1,2-diol + H2O + NADP(+) a 2-all-trans-polyprenylphenol + H(+) + NADPH + O2 = a 3-(all-trans-polyprenyl)benzene-1,2-diol + H2O + NADP(+) Cofactor biosynthesis; ubiquinone biosynthesis. Homotetramer (Probable). Component of the Ubi complex metabolon, which regroups five ubiquinone biosynthesis proteins (UbiE, UbiF, UbiG, UbiH and UbiI) and two accessory factors (UbiK and the lipid-binding protein UbiJ) (PubMed:30686758). Cells lacking this gene have a low level of coenzyme Q8 in aerobic conditions, and accumulate a compound derived from the Q biosynthetic pathway which was identified as 3-octaprenyl-4-hydroxyphenol. When grown anaerobically, they have a content of Q8 comparable with that in wild-type cells and do not accumulate 3-octaprenyl-4-hydroxyphenol. The levels of the isoprenoid naphthoquinones, demethylmenaquinone and menaquinone (MK8) are not affected in the deletion mutant under aerobic conditions. Partially complements the C5-hydroxylation defect of S.cerevisiae cells lacking COQ6. Belongs to the UbiH/COQ6 family.