Question

Why hot water contains less oxygen?

Answer 1

Oxygen is only slightly soluble in water.
At higher temperature, the O2 molecules have greater amount of kinetic energy than that in cold water. So the gas molecules in hot water can more easily overcome the weak binding forces within water and escape through the surface





Or

Answer 2: In general, hot water dissolves fewer gases (like oxygen or carbon dioxide) but more solids (like salt or sugar) than cold water does. Gases are more likely to escape into the air at high temperatures, and they are not replaced (in other words, re-dissolving into the water from the air) as quickly. In chemistry terms, we say the equilibrium has shifted toward the gas phase from the liquid phase.