Last weekend I noticed a story in the Herald about an 'oxygen bar' - somewhere where one pays to breathe in 95% oxygen for a fixed period of time. All sorts of health claims have been made for this, & it was good to see prominence given to the scientific viewpoint: that most folks' blood will already be saturated with oxygen, so breathing in the pure gas will not make much difference to them. In addition, any extra O2 boost that might occur would be lost in a couple of breaths, so long-term benefits aren't all that likely. (Also, breathing in oxygen at that concentration can be dangerous for those with some health problems.)
The story reminded me of something I read about a few years ago (again in the newspaper, but easily found in internet ads): "liquid oxygen" as a health boost. Since oxygen turns into a liquid at -183 degrees C, I was fairly sure people weren't chugging that down! The stuff turned out to be "charged stabilised oxygen", which must be the real deal as it was apparently invented by NASA** (</snark>): basically salty water with 15% 'bioavailable' oxygen. It comes in a handy 250ml bottle & the purveyors recommend taking 10-20ml half an hour before exercising, when you will then be able to train harder for longer. (There's also another 'liquid oxygen' dietary supplement, which it seems is based on a solution of hydrogen peroxide... yikes!)
Trouble is, as with the oxygen bars, it's hard to see how this stuff could have any immediate effects, let alone the long-term ones claimed for it, partly because there's no plausible route by which it could get to where it's needed. If your haemoglobin is already saturated with oxygen, then drinking a couple of spoonfuls of 'liquid oxygen' isn't going to make any real difference there. Not even if you had gills in your gut.
** Strangely, a search of the NASA website using the phrase as a search term didn't turn anything up...