[lmm]

Two generations of stars, including heavy stars that produced all the elements up to iron (including, relevant to this particular case, oxygen), and then became supernovae, which a) produced traces of heavier elements b) blew a lot of stellar material out into the universe, to become part of new star systems. So there was oxygen around by the time Earth was formed (~4.5 billion years ago, as opposed to ~14 billion for the big bang).