Involved in the biosynthesis of the antibiotics oxytetracycline and tetracycline. OxyS starts by catalyzing the stereospecific hydroxylation of anhydrotetracycline at C(6) position to yield 5a,11a-dehydrotetracycline (12-dehydrotetracycline). If the released product is captured by OxyR, it is reduced to tetracycline. However, if the released product is recaptured by OxyS, it performs an additional hydroxylation at C(5), producing 5a,11a-dehydrooxytetracycline, which, following the action of OxyR becomes oxytetracycline. 5a,11a-dehydrotetracycline + H(+) + NADPH + O2 = 5a,11a-dehydrooxytetracycline + H2O + NADP(+) anhydrotetracycline + H(+) + NADPH + O2 = 5a,11a-dehydrotetracycline + H2O + NADP(+) Binds 1 FAD per subunit. Antibiotic biosynthesis; oxytetracycline biosynthesis. Monomer. Belongs to the PheA/TfdB FAD monooxygenase family.