I saw on Discovery or Learning channel, I think, that it is CO2 in most balls. The rubber is porous enough for O2 to pass easily. Regular air will pass back and forth as you pressurize it in a tank. Eventually, the harder to pass molecules, N2 and CO2, will not bother to go into the ball. This will take a faster charge, but die out before you are finished with practice. When you recharge, consider other gases. N2 is the best, CO2 is second and regular air is the fall back. Never use O2, it just bleeds back out too fast.
As far as what happens on the balls in a tank? At 14 PSI of CO2, they will stay at their normal hardness. If they stay long enough, some CO2 will get inside. At 30 PSI of CO2, It will charge faster. Maybe as fast as 3 days. It will push CO2 into the ball, not letting O2 out. If you let it sit for a week or 2, the O2 will come out on its own and it will have more CO2 in the ball.
If you don't have a tank, get one. They are great, unless you are some major player that kills the felt every time you open a new sleeve.