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.”