Catalyzes the oxidation of glycolate to glyoxylate, with a reduction of O2 to H2O2. Is a key enzyme in photorespiration in green plants. glycolate + O2 = glyoxylate + H2O2 Binds 1 FMN per subunit. Photosynthesis; photorespiration; glycine from 2-phosphoglycolate: step 2/3. Homotetramer. Belongs to the FMN-dependent alpha-hydroxy acid dehydrogenase family.