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In Colorado outdoor stores its not uncommon to see these small canisters of 95% percent oxygen that's sold to help people cope with high altitude.

Is there any evidence that these actually help with altitude sickness? I mean plenty of places still sell the Sawyer Venom extractor kits and there is no evidence those work.

There is some evidence that increasing the concentration oxygen in the air breathed can help with mild symptoms of altitude sickness.

Source here looks at long-term increases in oxygen concentration at high elevation and finds benefits--whether or not the same happens with short bursts from a canister is unclear. I have heard circumstantial evidence from friends in the skiing world that a hit from the oxygen canister works pretty well for mild exhaustion after a hike at 13,000 ft or so.

Since one of the main causes of altitude sickness is the reduced partial pressure of oxygen at elevation, increasing the ratio of oxygen to other gases in the air should help a bit, to an extent. Of course, when the pressure is really low, no matter how high the concentration or O2, the partial pressure will never be enough.

I'd also add that the source in Gabriel's answer is about hyperoxia at sea level, not how to mitigate hypoxyia at elevation. Then again, Gabriel is probably right that stores carry them because they sell, not because they necessarily work.

Edit re: how many breaths is the canister "useful" for: At sea level where atmospheric pressure is 760mmHg and O2 concentration is 21% the partial pressure is about 160mmHg. By 10,000 feet, that drops to about 110mmHg. To maintain sea-level partial pressure of O2, you'd need to be breathing about 30% O2 instead of 20%, which means 1.5 times as much.

So for each 0.5L breath of 21% oxygen air at 10,000ft, you'd need to inhale an additional 0.052L of 95% oxygen to get the same inhaled partial pressures as sea level. The 10L can would give you 192 hits of this volume. This fits nearly with the 200 breaths advertised!

Obviously, at higher elevations, you'd need more oxygen, but since these are mostly sold at ski resorts where 9,000-12,000ft is the norm, the 200 advertised servings per container seems reasonable.

In this study from 2014 in the Journal of Strength and Conditioning Research, the conclusion seems to be that it's no better than placebo:

Introduction (excerpt)

Oxygen supplementation has often been used as an aid for athletic individuals who ascend to high altitudes before acclimatization or experience symptoms of altitude sickness. The effectiveness of oxygen supplementation in augmenting arterial blood oxygen levels [...] could have contributed to hyperoxia supplements, gaining popularity in the ergogenic aid market. [...] However, the efficacy of such products is in need of evaluation. There is increasing need for additional research to determine the efficacy of such products due to the fact that hyperoxic supplements are being made available to the general public, with promises of enhancing exercise performance and recovery.

Conclusion (excerpt)

As was previously stated, hyperoxia supplements are gaining popularity in the ergogenic aid market with a number of manufacturers supporting the use of personal oxygen cylinders to augment sports performance, and the aim of this study was to explore the efficacy of such products. The present investigation does not support the use of personal OSs for exercise performance, exercise recovery, or postexercise cognitive performance. It is important that strength and conditioning professionals are armed with accurate and relevant data when making decisions regarding the use of supplements. Given the results of this study, personal hyperoxic products do not seem to provide the desired results that would be expected from an effective ergogenic aid. Athletes, coaches, trainers, and recreational exercisers are encouraged to consider the results of this study when considering the use of similar supplements.

I am a big skeptic of this kind of "supplement". It seems like a good way to upsell cans full of worthless product, similar to homeopathic pills.

Usually, the reason stores carry these even if there's no proven health benefit is because there is demand, and they are harmless in themselves.

While not sure about the medical backing, from personal experience I once had HAPE and my oxygen reached 62% SpO2 by the time I was at the ER. I had one of those style canisters and I think it definitely helped during the 45 min between buying it and getting to the ER. I was using it fairly constantly during that time period though not just a single air shot as you see advertised. While I can't say conclusively what my SpO2 level would have been, it possibly would have been lower than 62%.