Bonjour Susan ! In this sentence, it's because we're considering that oxygen is not a countable thing (i.e. "Plants produce some oxygen."), therefore you need to use the partitive article, i.e. de l' . However, I agree that in this case, plants produce oxygen in general, therefore it would be more accurate to say : "Les plantes produisent l'oxygène.". Thanks to you, I edited the example. Merci et à bientôt !