Recently, the Rosetta mission found oxygen on the comet 67P/Churyumov-Gerasimenko. According to theories, oxygen should not be in the state in which it is on the comet. What are the previous theories that didn't support this, and what is the explanation for the oxygen-rich environment? How will this change our understanding of the formation of the Solar System?
It's not that oxygen should not be in this state, but rather we had not discovered it (e.g. on other comets). One of the reasons for this is that interstellar O2 is very reactive and quickly reacts to form e.g. H20 and O3.
Current models also favoured O2-less comets.
Interestingly, the paper you're referring to also mentions the O2 abundance was rather constant over the half year of observation, which hints to the O2 actually being primordial, i.e. from when the solar system condensed.
To reconcile these observations with models, a couple of possible solutions are offered: a slightly warmer primordial cloud (from which the solar system formed), or reactions with ices.