Catalyzes the oxidation of glycolate to glyoxylate, with a reduction of O2 to H2O2 (PubMed:21828292, PubMed:25416784). Is a key enzyme in photorespiration in green plants (Probable). Glycolate is the preferred substrate, but to a lesser extent, this oxidase is also able to use hydroxypyruvate, L-lactate and glycerate as substrates in vitro (PubMed:21828292). glycolate + O2 = glyoxylate + H2O2 a (2S)-2-hydroxycarboxylate + O2 = a 2-oxocarboxylate + H2O2 (S)-lactate + O2 = H2O2 + pyruvate kcat is 30.10 sec(-1) with glycolate as substrate (at pH 8 and 30 degrees Celsius). Photosynthesis; photorespiration; glycine from 2-phosphoglycolate: step 2/3. Homotetramer. A number of isoforms are produced. According to EST sequences. Belongs to the FMN-dependent alpha-hydroxy acid dehydrogenase family.