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Water Movement
Most dissolved oxygen is added to water by mechanical means. When water is splashed by waves, waterfalls or rapids, oxygen from the air above dissolves into the water. Aquarium owners place bubble stones in fish tanks so some of the oxygen from the bubble stone is captured and dissolved into the tank water.
Still bodies of water like lakes or dead-end canals have very low levels of DO, while a white-water river would have very high levels.
Plants
Another source of dissolved oxygen is the same source of oxygen above the water -- plant life. Healthy plants use photosynthesis to turn carbon dioxide into oxygen and sugar. The plants store the sugar for their own use, but the oxygen is a waste product to the plant. If the plant is in the water, the oxygen is dissolved into the water.
Water Temperature
Cooler water can hold more dissolved oxygen than warm water. If two bodies of water have the same parts per million of dissolved oxygen, the warmer water will have a higher saturation percentage than the cooler water. If the water gets too warm during the course of the day or summer, excess oxygen will be "outgassed" or released back into the air.
DO levels can vary quickly in shallow bodies of water that are fully exposed to the sun like small residential canals or farm ponds.
Decaying Organic Material
When plants and animals die and fall to the bottom of a lake they decompose. This decomposition process is done by bacteria which consume dissolved oxygen to do their work. If a body of water has a lot of decaying organic material in it, the bacteria can quickly deplete oxygen levels.