It intends to launch the next-generation service in Belfast, Cardiff, Edinburgh, London, Slough and Leeds and then expand to a total of 50 towns and cities by summer 2020.
It will be the last of the UK's network coverage providers to roll out 5G.
However, it will be the only one to do so without using equipment from the embattled Chinese telecoms equipment-maker Huawei.
O2 is owned by Spain's Telefonica, which has used Huawei's infrastructure in some of its other networks.
Moreover, O2 trialled some of Huawei's 5G radio access network gear at cell towers in the UK before deciding to opt for rival products from two vendors it had already used to deliver 4G.
"We respect all three operators, they were thorough in their submissions," O2's chief executive Mark Evans told the BBC.
But we were convinced that the best choices for us at this time are our current partners, which are Ericsson and Nokia.
The announcement comes the same week that the UK formally postponed a decision on whether to ban or allow Huawei to be used within any of the UK's 5G networks.
The US has been putting pressure on the government to exclude the Chinese firm claiming that it poses a risk to national security - something that Huawei denies.
In April, it had seemed that former Prime Minister Theresa May had decided that any threat could be managed.