Note: the oxygen concentrator must be at least 20psi if using it with a 1.3 ATA chamber for me to recommend a model and brand. If the flow is less than 10L per minute with a 1.3 ATA hyperbaric chamber it will still come out but not enough oxygen for many people. If doing EWOT or the person is going to be "burning" oxygen and not just lying down in the hyperbaric chamber, a higher flow can be recommended. If using a 1.5 ATA chamber make sure the psi is strong enough for the air to enter the chamber.
Note: There are many different models that have a back pressure of 20 psi and 10 liters per minute. The Airsep is my favorite model as I have had the least issues with this brand. There are other brands that work just as good but they are louder and vibrate my floor more.
The Airsep Intensity is the oxygen concentrator I use. The manufacturer is located near me so I can pick them up and get a really good deal on them. This is the only brand I am comfortable selling.