Chlorophyllide-a oxygenase (), chlorophyllide a oxygenase, chlorophyll-b synthase, CAO) is an enzyme with systematic name chlorophyllide-a:oxygen 7-oxidoreductase. This enzyme catalyses the following overall chemical reaction
The oxidation reaction uses molecular oxygen with nicotinamide adenine dinucleotide phosphate (NADPH) as a cofactor.
This enzyme contains a mononuclear iron centre and is part of the biosynthetic pathway to chlorophylls.