Why do some rockets carry oxygen in addition to fuel?

There is no oxygen in space, and if you were to not have any oxygen you'd die.

if your in space in a rocket your going to need oxygen because if you run out of oxygen there are tanks to release more into the rocket :D

beacause the people could get lost and loose thair helmet so they will get some oxygen if they take it

Because of the weight difference. Fuel weighs 8.66 lbs while oxygen weighs 16g. The weight matters because of the speed the rocket has to go. Fuel may slow it down. And oxygen can speed it up.