The purest oxygen comes from a tank with a regulator attached. The purity is approximately 98% pure O2.
The second best purity comes from an oxygen concentrator. Purity ranges from 83 to 95% depending on the quality of machine, hours on the machine, and flow rate of oxygen.
According to a friend of mine in the concentrator rebuilding business, the best ones are made by Airsep, the second best and most common are by Mobilaire (Invacare)and another is DeVilbis. These units generally produce from 1/2 to 5 litres of oxygen per minute but some units produce up to 10 lpm.
There are UV bulb units which attach in the plenum of the HVAC unit which not only kill microbes on contact, but also give off up to 35 mg of ozone for air purification of all the ducting and the entire house. The bulb runs when the blower is activated. You can see them on our website in my profile.