Meh. It's not a completely bad episode, the first twenty minutes or so are somewhat entertaining even if a bit "been there, done that." But when we get the reveal of the dangerous solar flare mere minutes after learning the trees are fireproof, it becomes very obvious the trees are meant for protection. But the episode drags its ass trying to makes us believe the solar flare spells doom that it makes the second half rather tedious indeed. And how does it take a genius over two thousand years old like the Doctor so long to figure out the blindingly obvious. This isn't a case where the Doctor is intentionally playing up his ignorance for comedic effect, as when he does figure things out, he seems genuinely surprised. "Doctor Idiot" or whatever it is he calls himself indeed. Worse, even Missy seems surprised at the outcome. Maybe she missed the revelation the trees are fireproof?
Just because that was an obvious possibility doesn't mean it's the only one. The trees being fireproof could have simply been their own survival mechanism for the coming apocalypse (or something done to protect the Earth itself). It doesn't automatically mean they were out to protect humanity as well, or that such a thing would even be possible.
Until it was brought up at the end, there was nothing to suggest this so-called "oxygen barrier" would have even existed or been able to help.