Hi and Welcome to the Forum.
No you cannot do this online.
Help links :
To request a PAC Code to keep your number to migrate to another Network ->
http://community.o2.co.uk/t5/Discussions-Feedback/Obtaining-a-PAC-Code-from-the-O2-Network/m-p/54427...Guide to Cancelling ->
http://community.o2.co.uk/t5/Pay-Monthly-and-Pay-Go/Cancelling-Your-Contract-A-Guide/m-p/566774/high...If on a standard Contract it is the amount of months left to run ,minus any bolt ons and O2 insurance if applicable.
If on Refresh Tariff you pay your Handset Plan off and tell O2 on 202 you wish to cancel the airtime plan.
Customer Services on 202 will confirm the amounts.
Please do not cancel any O2 Direct Debits for a few months after you've paid the final bill just incase O2 try and take any further payments because if they do, even in error, this could result in a default payment followed by them chasing you ,and in the extreme debt collectors and default markers on your credit file.
Tip :
If your phone is locked to O2 then perhaps unlock it so you can either use it on another Network or increase its street resale value. It's free to contract customers.
Form ->
https://www.o2.co.uk/apps/help/help?qid=1&q1=2&route=unlocking&case=Handset%20Unlocking%...Please check out any alternative Networks via their payg SIM cards to check on signal where you live, work and commute. Do not go on online coverage checkers.
