Trains from Stansted run into Liverpool Street station. You could use Stansted express. [
www.stanstedexpress.com]{B13F439D-D95C-47D6-8C30-E20558FB48B6}&pageid=73
From Liverpool Street take an eastbound (Barking) Metropolitan Line service to West Ham, then change onto the Jubilee line to North Greenwich for the O2.
See here. [
www.nationalrail.co.uk]
There you can also find how to get to Luton from central London, former Thameslink service now First Capital Connect (ctrl+F "luton" if you don't see it right away.) Plan here: [
www.nationalrail.co.uk]