It's not O2: if you let water sit out, various things (including CO2) dissolve in it, making it slightly acidic and taste stale. I'm not a big tea drinker, but

It's not O2: if you let water sit out, various things (including CO2) dissolve in it, making it slightly acidic and taste stale. I'm not a big tea drinker, but I am definitely not a super fan of water that's been sitting out a day. There's not been time for stuff to grow in it yet, but it still tastes gross.