I used to work for a very large bottled water company. Those 5 gal carboys will work fine. You would have to leave your beer in one for many, many, MANY months before you would see any oxidization.
Bottled water will pick up off flavors very quickly, that's why it has a shelf life. The shelf life for most 5 gal water bottles is at least 6 months, and could be up to a year.
I like better bottles because they hold more (6gal), but would not hesitate to use a good, clean 5 gal plastic water carboy again.
Now, go brew some beer.......