Catalyzes the oxidation of glycolate to glyoxylate, with a reduction of O2 to H2O2. Is a key enzyme in photorespiration in plants. To a lesser extent, is also able to oxidize glyoxylate to oxalate in vitro. Can exert a strong regulation over photosynthesis, possibly through a feed-back inhibition on Rubisco activase. Does not seem to play a role in oxalate accumulation