"It is certainly counter-intuitive at first that you can have gas flow in a reverse direction to the pressure gradient (e.g. the pressure inside the bottle is ~3 atm, while the pressure outside is ~1 atm), but it does."
“Over a 6 month period a typical crown cap will allow approximately 1500 ppb of O2 to enter the bottle. Bottles with more than 600 ppb O2 will show definite signs of staling in 6 months. The Oxygen barrier cap limits the O2 ingress to (if you do not sanitize, as the sanitizer and water will have O2 negating any positive effects.) ~125 ppb over the same 6 month period (at 24°C).” (Data from the Proceedings of the European Brewing Convention held in Oslo 1993, pg 654)."
http://www.********************/brewing-methods/bottle-cap-oxygen-ingress-real/
Cheers!