I'm not sure what the global O2 levels are, but there's nothing to worry about. People can easily survive on less than half of the current levels...and they do it all the time.
The normal atmosphere is about 20% O2, and normal sea level pressure is around 30.00 in Hg. People living/travelling above 18,000 feet live with about 15.00 in of pressure, and presumably the same ratio of O2. Thus, half the O2. I've climbed to 19,300 feet and was able to breath just fine. Who am I kidding, it was very hard to breath, but I survived. Mountain climbers know that 8000 meters is the magic point that cannot sustain life with out Oxygen. You can go above that altitude without O2, but your body is in a constant state of decay. Standard pressure at that alitude is about 10.00 in Hg. Its amazing what the body can adabt to.