The following may help increase dissolved oxygen:

 Your waterfall pump.
 If using a water pump, a venturi tube.
 The longer the oxygen bubble is in water the greater the dissolved oxygen. A deep reservoir will help do this. However, this depends on whether your pump has enough draw for a deep reservoir.
 Hydrogen peroxide.
 If you are going with a NFT system flow rate & system slope affects root 02. Steeper slopes & a faster flow rates are recommended. There is a bit of information on this page:

http://www.usu.edu/cpl/research_hydro6.htm

Go to:
DESIGNING HYDROPONIC SYSTEMS: THE IMPORTANCE OF FLOW RATE

The Sri Lanka Agriculture site has some interesting info on NFT slopes:

http://www.gov.lk/Agriculture/Agridept/Techinformations/HPindex.htm

Key word slope:

"The maximum length of the channel is 5-10 m and is placed at a slope drop of 1 in 50 to 1 in 75." (1m is about 1 yard).