I know this is somewhat tongue-in-cheek, but what would the long-term consequences be of importing large quantities of hydrogen from off-planet and then binding

I know this is somewhat tongue-in-cheek, but what would the long-term consequences be of importing large quantities of hydrogen from off-planet and then binding it to local oxygen? I suppose if we had the tech to do that, we might be able to find off-planet oxygen as well...? Except wasn't most of ours produced by plants splitting carbon-dioxide?