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.

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem