For a time after the shift, the clouds will be lower, as the atmosphere will be ripped away. This will if anything create a sense of being on a higher elevation, less air pressure, but you will be so preoccupied that you won't notice. The oxygen availability will also not create any more problems than the average city presents to you, due to pollution. Soon, within years, less than a decade, the oxygen will be replenished due to ocean kelp and the like. As we have stated, if you are not utterly out in the open, in a semi-enclosed environment, during the shift, you will not suffocate.