There are numerous threads about people being charged (usually 31 per day) by O2 for data usage even though they are using wifi.
The problem here is that you will almost certainly have your cellular data turned on. Even if you are not using your phone, the cellular data can still run in the background as many apps will run or update in the background and overnight.
You can turn your wifi and cellular data on and you will get charged for data even if you are browsing the internet and using apps through wifi.
All phones are different but you can find your cellular data options in the settings. Disable cellular data to turn off the cellular data and allow you to run your phone only through wifi.
You cannot get refunded from O2 for these charges as it is the users fault for having their cellular data turned on.
The best thing the user can do to prevent being charged for data usage (and also to preserve battery life) is to have one or the other on. So if you at home and you have a wifi signal then use only wifi and keep the cellular data turned off. Or if you are at the shops and there is no wifi signal available then turn wifi off and turn cellular data on. It is common sense really but sometimes people do not understand that the phone will still run off both and not just to wifi until you turn cellular data off.
