Furious customers took to social media to flag problems with 4G and data services.
It appears that the network issues started about 5.30am, and customers across Manchester, London and Southampton have all reported outages.
The network confirmed they were investigating.
A statement posted on the company's website read: "Our technical teams are investigating reports of issues when using data. Voice calls are working OK.
"We apologise for any inconvenience."
It also issued a statement on social media, commenting: "We are aware our customers are unable to use data this morning.
"Our technical teams are working on the issue with high priority.
"We are really sorry and working as hard and as fast as we can to fix this.
"Please keep an eye on our status checker: o2.uk/ServiceStatus."