Thank you for the opportunity to help you with your question!
Catechol is an enzyme which is involved with ripening of fruits, especially the color. This happens when Catechol enzyme is oxidized, or as you said, reacts in the presence of oxygen. when it reacts, catechol loses hydorogen and electrons under the catalysis of catechol oxidase. Remember catechol is an enzyme, when oxygen reacts with it, it becomes a pigment compound after losing the hydrogen and electrons. this pigment compound is known as Benzoquinone and it is the one responsible for coloration of ripe fruits eg the yellow bananas and purple passion fruits. That is what happens when catechol reacts in precense of oxygen.
Hope this did assist. Please let me know if you need any clarification. I'm always happy to answer your questions. I will be pleased if you make me your private tutor.