An oxygen tank, fitted with a regulator can feed pure oxygen into the wort prior to pitching. It is usually diffused using an aquarium air diffusing stone or the more expensive stainless steel version available at home brew shops. The oxygen tank comes in many forms. Larger tanks are used in industrial cutting and welding. Medium sized tanks are used to dispense oxygen to patients with certain medical conditions. Finally, the smaller disposable tanks are sold for light duty torch applications at hardware stores for about US$9.
This video show a typical setup with a used medical tank: http://www.youtube.com/watch?v=EJwaOKm4HFM