Catalyzes the 3-hydroxylation of L-asparagine to (2S,3S)-3-hydroxyasparagine. The 3-hydroxylated asparagine produced is incorporated at position 9 during the biosynthesis of the non-ribosomally synthesized calcium-dependent antibiotic (CDA), a 11-residue acidic lipopeptide lactone. Is able to hydroxylate only free L-asparagine, since it hydroxylates neither a CDA analog with unmodified Asn at position 9 nor a peptidyl-carrier-protein (PCP)-bound asparagine. Is not active toward D-asparagine. 2-oxoglutarate + L-asparagine + O2 = (2S,3S)-3-hydroxyasparagine + CO2 + succinate Binds 1 Fe(2+) ion per subunit. Antibiotic biosynthesis; calcium-dependent antibiotic biosynthesis. S.coelicolor strain A3(2) / MT1110 and A3(2) / 2377 lacking asnO produce non-hydroxylated asparagine-containing CDA variants, which are not synthesized by the wild-types but retain calcium-dependent antimicrobial activity. Belongs to the clavaminate synthase family.