What would happen if our atmosphere consisted of pure oxygen?

You mean, biologically? There would be no life, at least not as we know it. Plants need carbon dioxide. I suppose an ecosystem composed entirely of animals might be possible. But animals produce CO2, methane, and a host of other off-gases. So your hypothetical planet would be populated by things very different from us. 

Well too much oxygen can kill humans which is one. And 2 If the whole atmosphere was oxygen then things will burn easily.