Furious customers of the network, which has a total of 32million customers and is the second largest mobile network in the UK, have taken to social media to flag problems with 4G, data services and phone calls.
The network issues started about 5.30am with around 1,662 complaints made before 7am, and customers across Manchester, London and Southampton have all reported outages.
But the issues have also spread beyond England to Scotland and Northern Ireland, a recent heat map by the Down Detector shows.
The latest update was from 02 boss at 4pm, he tweeted to apologise for the outage and that staff are still working to find a "swift resolution".
The network, which is also used by customers with Tesco Mobile, GiffGaff, Sky Mobile and Lycamobile, has blamed the issue on faulty software provided by third party supplier Ericsson.
The Swedish company has also issued a statement saying it believes the issue was caused because by "an expired certificate in the software" installed with O2.
Yet spokespeople for both O2 and Ericsson have told The Sun they cannot yet say when the issues are going to be fixed.
O2's latest statement says: "The network issue we are currently experiencing remains a top priority for us and we are working as quickly as possible to fix the issues.
"All O2 technical teams are working closely with one of our third party suppliers who has identified a global software issue in their system which has impacted data services.
"As a result of this incident, we are experiencing a high demand on our network which means some customers may have issues making and receiving voice calls.
"We apologise to our customers for any inconvenience."
O2 customers can check for updates on O2.uk/ServiceStatus.
Until the issues have been fixed, O2 also encourages customers "to use wifi wherever they can."
Meanwhile, Börje Ekholm, president and CEO of Ericsson, said: “The faulty software that has caused these issues is being decommissioned and we apologize not only to our customers but also to their customers.
"We work hard to ensure that our customers can limit the impact and restore their services as soon as possible.”
Mark Evans, CEO of O2's owner Telefónica, has also tweeted that the company is "working really hard with Ericsson to find a swift resolution".
Apart from customers with O2, Tesco Mobile, GiffGaff, Sky Mobile and Lycamobile, the outage has also affected systems relying on the same network such as the electronic timetables for London's buses and Boris Bikes due to their O2 sim connections.