Electronegativity and electron appetite
Definition
Each oxygen atom is highly electronegative, giving the molecule a strong thermodynamic appetite for electrons even though it is covalently satisfied.
Covalent satisfaction within the O2 molecule does not cancel the energetic eagerness of its atoms to accept electrons if the opportunity arises. This appetite is the thermodynamic reason oxygen products are energetically favorable. It explains why oxygen remains a persistent oxidizer despite looking finished.
Related concepts
Kinetic restraint of oxygen · Molecular oxygen as default oxidant
Part of The Blueprint of Water · 🔥 thermodynamics