Some customers have reported being unable to use their data, while others are struggling to send texts.
The company said one of their third party suppliers has identfied 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.
O2 said voice calls were still working, however many customers have reported not being able to use their phones at all.
The problems mean that the companies that share its network – such as GiffGaff, Sky Mobile and Tesco Mobile – are also being hit by the problems.