[bongodongobob]

The didn't make the oxygen, they unlocked it from various molecules and minerals etc. It had to be here first.

[greiskul]

But I don't think there is anything special on there being oxygen in the cloud of gas that formed the solar system. The distribution of elements following a supernova probably follows some law, and oxygen is low enough on the periodic table that probably quite a bit of it is expected.

[czzr]

True - oxygen is very abundant, though, so its presence is not surprising.