Oxygen is controlled in water by forcing oxygen into the water. There are typically three ways to accomplish this:
Place a paddle-wheel type device that mixes the water so that more water comes into contact with the air. This process is used in open pond systems. This can only increase the amount of dissolved oxygen in the water to be in equilibrium with the atmosphere.
Force pressurized air (or pure oxygen) into the water with diffusers.
Allow the water to free fall through the air. This process releases any over-saturated gases and increases the surface area of the water so that more dissolved oxygen in the area can be exposed to the water. Control the mechanical equipment that forces more oxygen into the water. In open ponds, this is traditionally done by turning on paddle-wheel aerators. In closed re-circulating tank systems, this is done by turning on additional aerator pumps.