Cold water holds more oxygen than warm water because water molecules are closer together in cold water, which makes it harder for oxygen molecules to escape. The tighter structure is also conducive to a more consistent attraction between the oxygen and water molecules.
The solubility of oxygen in water depends on many factors. It has been shown that oxygen becomes more soluble in water as it gets colder. Oxygen is also more soluble in water with a lot of surface area. Water that splashes over a rock, like in rapids, will have a larger surface area and is likely to contain more oxygen than still water.