In the context of "breathing oxygen" in a normal earth surface (or higher) environment, as I took the meaning of the original question, the commonly accepted rule is that oxygen is not toxic for most people.

Premature infants, given pure O₂ are prone to damage to central nervous system, lungs, and to eyesight. In this case the oxygen was typically forced under slight positive pressure, usually in a breathing apparatus resembling a mini-iron lung. The infants who need assistance, and who are most likely to suffer adverse consequences, generally have "undeveloped" lungs and other organs. The use of pure oxygen for neonatal care was discontinued in most places at least a couple of decades ago, primarily due to oberved damage to the eyes which was more obvious than other adverse results that have been documented since.

Under hyperbaric (high pressure) conditions, as in treatment chambers¹ for "bends" or during actual activity under water beyond fairly moderate depths, there are well known adverse effects of using pure oxygen, or even of use of simple compressed air. For avoidance of bends, the critical factor is the removal of Nigrogen from the mix, and for deep underwater activities there are a number of other gaseous mixtures, some proprietary, that are generally used. Quite a few cases have been cited for use of Helium/Oxygen mixes, but other gases have been developed that give better results with fewer problems. No diving requiring any of these "special provisions" should ever be done without qualified medical observers and immediately available "decompression facilities."

My prior reference to lack of "toxicity" of pure O₂ were not intended to reflect hazards that might occur under abnormal or extreme conditions, or to include all possible medical uses,as that was not a condition I interpreted the original question to include.

Foolestroupe is correct about the use of pure O₂ in early US space vehicles. An original primary reason was to avoid the necessity of carrying an additional "diluent" gas. The Russians reportedly continued using pure O₂ internal atmospheres in their space vehicles for some years after the US switched, and contention over that use was significant when the first "joint missions" were considered. At least for a time, in some early joint missions, the US accepted the Russian usage when US astronauts were onboard Russian vehicles. I don't know what Russian astronauts are using now, or whether it's different for their own missions than it is for shared missions.

Clinical research on the use of pure (or "enriched") O₂ as a "hangover cure" have found little physiological benefit, and most who've been involved in such tests apparently believe that any effects are "mostly psychological." IF you are suffering from low pO2 a moderate "better feelingness" might be noticed. A low pO2 could be caused either by reduced absorbtion of O₂ or by presence of other "contaminants" not fully removed by normal renal processes. Some byproducts of alcoholic ingestion can reduce the carrying capacity for O₂ in the blood and some others can inhibit both/either normal absorbtion or release of O₂ by the hemoglobin. In either case, to get more oxygen to the tissues, you'd have to change the amount of hemoglobin available to carry it. Adding more oxygen in the lungs doesn't make a significant difference in normal people. Inhaled O₂ in any concentration won't "burn out the crud" and according to test results I've seen has no significant effect on the rate at which "afterproducts" are removed from the system. Oxygen enriched breathing might help if your breathing is "depressed," but in that case you don't have a hangover, you're still drunk.

¹ Hyperbaric O₂ treatment has been used some for treatment of a few special conditions not related to "bends" or other underwater problems. A most notable usage has been for treatment of very severe burns. While I haven't seen many detailed studies of the effectiveness, treatment facilities using them claim good results. Adoption of the method has been rather less than I would have expected if the results were as much better than more conventional treatments as claimed; but it's been some years since I've seen good numbers. Any use of this kind must of course be under the supervision of qualified specialists.

John