The company said it would be closely monitoring data services over the coming days and promised to carry out a review to understand what went wrong.
British customers reported not being able to use mobile data to access the internet and the operator’s network on Thursday after disruption began at about 5am.
On Thursday evening, O2 said 3G data service had started returning and was expected to be fully restored by 9.30pm, while the company reported at 3.30am on Friday that the 4G network had been restored.
“Our technical teams will continue to monitor service performance closely over the next few days to ensure we remain stable,” a spokesman said.
“A review will be carried out with Ericsson to understand fully what happened.
“We’d like to thank our customers for their patience during the loss of service on Thursday 6 December and we’re sorry for any impact the issue may have caused.”
The company had earlier issued a joint apology with telecoms company Ericsson.
O2 UK chief executive Mark Evans said: “I want to let our customers know how sorry I am for the impact our network data issue has had on them.
“We fully appreciate it’s been a poor experience and we are really sorry.”
O2, which has more than 25 million UK customers, saw disruption to its network last for most of the day on Thursday.
Other mobile networks, including Sky, Tesco and Giffgaff, were also affected by the problem because their networks use O2 services.
Marielle Lindgren, chief executive of Ericsson UK and Ireland, said: “The faulty software that has caused these issues is being decommissioned.”
She added: “Ericsson sincerely apologises to customers for the inconvenience caused.”
What are your rights about claiming compensation?
Ofcom says that, depending on the circumstances, it may be appropriate for your provider to offer you some money back while repairs are being carried out. In more extreme cases, where repairs take much longer (for example it takes longer than usual to access a mast site to undertake repairs), you may be entitled to an additional refund or account credit. In cases where you have been without service for some time, you may also have the right to leave the contract without penalty. There may be a term in your contract saying you can do this if your provider has failed in its obligations to you or breached a key condition.
How do I complain?
Ofcom says that if your provider fails to repair a fault by the date promised, or you are unhappy with how long it is taking, you should follow their formal complaints procedure. Details should be available through their website or customer services. If your problem is still unresolved after eight weeks, you can submit your complaint to an independent Alternative Dispute Resolution (ADR) scheme. If your problem cannot be resolved, ask your provider for a ‘deadlock’ letter so that you can refer your dispute to the relevant ADR scheme directly before the eight week mark. Ofcom has approved two ADR schemes – CISAS and Ombudsman Services: Communications.
For more information, visit the Ofcom website.