The term 'ADD OXYGEN' can be solved by two crossword solutions: AERATE (6 letters) and OXIDATE (7 letters). AERATE means to introduce air or oxygen into a liquid, usually to improve its taste or quality. OXIDATE means to combine with oxygen, often resulting in a chemical reaction that produces rust or other corrosion. Both terms refer to the process of adding oxygen to a substance, either through aeration or oxidation.