||Oxygen by itself is not really much of a psychoactive. Higher levels of oxygen than are present in normal air can feel refreshing and soothing. In recent years some "oxygen bars" have sprung up, like these in San Francisco http://www.2202bar.com/ and Las Vegas Breathe Inc.|
Andy Weil answered a question about oxygen bars last year, including some interesting bits of data and recommending people stay away from the fad. While he seems to be primarily concerned about the potential health issues, I doubt that breathing pure oxygen would be more damaging to the body than a night out drinking alcohol.
The three major components of the normal atmosphere are Nitrogen, Oxygen, and Carbon Dioxide. According to The Open Door Website's introduction to the environment, Nitrogen makes up 78%, Oxygen, 21%, and CarbonDioxide .03%. As we breathe, however, we increase the percent of carbon dioxide in our immediate environment and the level of localized carbon dioxide is usually much higher than the global 0.03%. In the lungs themselves, the levels of carbon dioxide get very high between breaths or when not breathing and continue to increase until carbon-dioxide sensors in the body send powerful signals to the brain, including pain and a feeling of suffocating. This signalling can trigger panic, anxiety, and elicit a wide variety of strong mental and physical reactions. The interesting point is that these signals are caused by an increase in carbon dioxide in the lungs, not a decrease of oxygen.
Theoretically, it seems believable that increasing oxygen levels could be psychoactive. By simply manipulating the ratios of carbon dioxide and oxygen, one can create a gas which will easily sustain life but will trigger the carbon-dioxide "suffocation" sensors. There is a gas combination called Meduna's Mixture or "Carbogen" which was used by some of the early psychedelic pioneers as a brief way of introducing powerfully altered states. The stories are too many to be told here, but this simple mix of Oxygen and Carbondioxide can induce totally overwhelming psychedelic-ish experiences. If increasing the level of carbon dioxide can cause powerful psycho-physical effects, then it seesm reasonable to think that decreasing carbon dioxide could have effects as well.
Another related topic is something called "nitrogen narcosis" which occurs when the normal atmosphere is breathed under pressure, most common with SCUBA diving. The effects of nitrogen narcosis are reportedly similar to the effects of nitrous oxide and is definitely powerfully psychoactive. While this does not occur at normal atmospheric pressures, it again points to the fact that the atmosphere we live in can affect our thinking.
More directly, oxygen is used by many people with serious illnesses or dysfunctions and is used in hospitals in the treatment of a variety of problems. I've also seen it given to people who were panicking at SCUBA diving sites as a sort of rapid tranquilizer. Many people report inhaling oxygen causes them to feel more at peace, more relaxed, and less anxious. It is possible, although untested and highly speculative, that some of the relaxing effect of being outdoors in natural environments might be caused by the difference in oxygen ratios.
Unfortunately it's extremely difficult to separate the effects of deep breathing from the direct pharmacological effects of increased oxygen. Simply breathing deeply can decrease anxiety, increase relaxation, and cause a mild mood lift. Its a hard question to answer without some double-blind experiments.
We visited an Oxygen bar in San Francisco (the 2202 bar above) after a few beers to hear a friend DJ and socialize early in 2001. The friends we went with gave it a try. The atmosphere in this O2 bar was very relaxed and trippy. They served herbal 'tonics', had no alcohol, and (per state law) did not allow any smoking inside. The aesthetic was decidedly yuppy-psychedelic: rotating rainbow prism wall sconces, optical illusion 3D art, odd sculpture, etc. It seemed to me that the choice of art, music, and atmosphere was intended to increase the sense that the Oxygen was going to be psychoactive. The method at this bar was that one paid 15 or 20$ or so to sit in a chair with a clear vinyl tubes and nosepiece feeding oxygen into one's nose. Customers were told to inhale deeply through their noses as they sat back and socialized with a steady, light pressure of oxygen blown into them.
They described feeling that in order to "get their money's worth", they wanted to breathe deeply and really give it a try. Soon after they started breathing the Oxygen, they described that their head felt clearer, they felt more awake, yet relaxed and calm. An interesting effect described was that their previous (mild) alcohol buzz was completely eliminated after 15-20 minutes on the oxygen. Overall they described their experience as a pleasant novelty and one they would consider repeating. We had already spent our day's money on beer and french fries, so we didn't give it a try, but I would consider going back again another time.
One EMT friend described that he and other EMTs, who had constant access to oxygen, would use it as a way to treat their own headaches. He said that breathing pure oxygen for a minute would eliminate their headaches.
Overall, I'd say that the experience of going to an Oxygen bar and doing some deep breathing is definitely psychoactive, but I think its an open question whether one would have the same experience if they just served up a standard air mixture. We have one report from a realiable source where The Spivey found that deep breathing and oxygen-breathing were equivalent, which can be found in the Experience Vaults.
Let us know if you give it a try and submit an experience to the Experience Report Vaults