What situation would reduce the dissolved oxygen content of water

You could do this in two ways:
1) Reduce the partial pressure of oxygen in the air that is over the surface of the water. You may recall that gases are more soluble in liquids at higher partial pressures.
2) Increase the temperature of water. Gases become less soluble in liquid as temperature is raised.

An interesting real-life application of this knowledge would be what happened to the dissolved carbon dioxide in Lake Nyos in 1986.

Hope the image of the frequency formula