Why can the same oxygen make an open bottle of red wine taste nicer tonight but worse tomorrow?

Answer: Oxidation keeps going

Oxidation keeps going ✓ — Correct. Oxygen is both useful and dangerous because oxidation is a process, not a switch. A little air can soften sulfur notes and open aromas; too much can flatten fruit, brown color, or create stale odors. The surprising lesson is that 'let it breathe' has a peak, and after that the same cure becomes the damage.

Room warmth alone did it — Not enough. Warmth can make wine smell heavier and can speed reactions, so room temperature may worsen an open bottle. But the main reason the same bottle changes overnight is that oxygen keeps reacting with wine compounds. Temperature is an accelerator, not the whole engine.

Aromas only evaporate — Partly, but too narrow. Volatile aromas do evaporate from an open glass, and that can make wine seem quieter. Overnight decline is more than evaporation: oxygen also creates new oxidation products and changes color and fruit impression. Losing smell and making stale flavors are different problems happening together.