The phone firm said the network was up and running again after the chaos that wreaked havoc for users across the UK on Thursday.
It comes after its CEO Mark Evans apologised to the millions affected by the huge outage.
In a statement on 02’s website, the firm since said the network had been restored early on Friday morning.
The statement read: “We can now report that our 4G network has been restored.
“Our technical teams will continue to monitor service performance closely over the next few days to ensure we remain stable.
02 added that a review would be carried out to understand fully what happened.
Apologising for the outage yesterday, Mr Evans said: that his teams were “working really hard” to find a “swift resolution” adding that he was “sorry to everyone affected.”
Bosses at Telefonica which owns O2 and Ericsson UK and Ireland had said they hoped to restore full service by Friday morning.
02, Britain’s second biggest mobile network blamed the issue on a third-party supplier and Ericson, which makes mobile network equipment.
A total of 32 million customers are thought to be affected with phones failing to show any data connection.
Customers using Sky, Tesco Mobile, GiffGaff and Lycamobile, which share O2’s network, were also hit.
London’s Santander Cycle network was also affected with customers saying there were unable to take out bikes due to malfunctioning terminals. Screens at London bus stops were also hit.
A TfL spokeswoman said: “We’re sorry that customers are unable to use our Countdown screens at bus stops for live travel information and some Santander Cycles customers cannot hire bikes.
“This is a result of a nationwide O2 data outage. We are working with our service provider to resolve this as soon as possible.”