Oxygen is actually pretty inert i.e. non-reactive and therefore safe to breathe in pure form (as aquatic divers do, I believe). The reason for that is most of a

Oxygen is actually pretty inert i.e. non-reactive and therefore safe to breathe in pure form (as aquatic divers do, I believe). The reason for that is most of atmospheric oxygen is composed of "triplet oxygen" referring to its outer shell electron configuration being rather stable. For reactions to occur triplet oxygen first needs to be activated and converted into "singlet oxygen" being rather aggressive chemically. This is a "kinetic" barrier that renders gaseous oxygen pretty harmless. This is also the reason why organic matter on earth does not spontaneously catch fire although CO2 + H2O is always energetically more favorable than every CXHXOX composition.