Make a name for yourself shooting small gigs, get lucky with some good pics, get someone with influence to get you a pass for the smaller venues and hope that they do the same for the O2. You won’t get one unless you’re working for someone, and even then there’s no guarantee that you’ll get in/get into the pit – you could be shooting from the balcony!
But if you want a quick way to do it, know that there isn’t one, unless you know someone VERY important, personally at the O2.