Imagine that humanity found a rocky planet with earth like gravity, atmospheric pressure, and temperature, orbiting a sun-like star. The only problem is that the planets atmosphere is that it is composed of 20% nitrogen, 60% hydrogen, 14% ozone, 3% argon, and 3% other gasses.
Assuming that the colonists to this planet had sufficient amounts of oxygen with them, could they breath a mixture of 80% the planets natural atmosphere and 20% pure oxygen safely? If not, what would be the effects of this natural-atmosphere+oxygen mix on the human body? Whether or not these gasses are a probable/viable atmosphere are unimportant for the scope of this question.