However the company said its 2G network had now been restored and that customers should now be able to make and receive calls reports The BBC.
The widespread issues began “at lunchtime” on Wednesday which meant many lost voice and data services.
No timescale has been given for when the network will be fully back up and running.
In a message published at 08:00 GMT on its network status site, O2 said: “We can confirm that our 2G network service has now been restored. Customers who were affected should now be able to make and receive calls.
“Our 3G service is starting to restore and customers should expect to see a gradual return of data services as the day progresses.
“Customers affected may wish to try switching their mobile phones off and on as service returns.”
It apologised again for the inconvenience that was being caused. No details have been given about what has caused the network problems.
Customers of other firms that use O2’s mobile masts, such as GiffGaff and Tesco Mobile, were also still affected.
Tesco confirmed that many of its users were also having problems and cited issues with O2’s “core network”.