Customers have been suffering problems since around 4am and many are still unable to make calls, send texts or use their mobile data.
Now in a tweet, Mark Evans, CEO of O2 owner Telefonica UK has said: "I want to reassure our customers that we are doing everything we can to fix the issue with our network and say how sorry I am to everyone affected.
"My teams are working really hard with Ericsson to find a swift resolution. Stay updated: https://status.o2.co.uk/."
The company said one of their third party suppliers had identified a software issue in their system. Work is currently ongoing to fix the fault and customers are being encouraged to use wi-fi where possible.
Companies that share O2's network – such as GiffGaff, Sky Mobile and Tesco Mobile – are also being hit by the problems.
Can I claim compensation from O2 after the network went down?
Ofcom say 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.
What have O2 said?
The company have so far not said anything about whether they would be giving customers compensation for the outage. However, when the network was down for seven hours in 2015, the company did not offer compensation to all customers, instead saying payouts would be assessed on a 'case-by-case' basis.