The operator has started its rollout in Belfast, Cardiff, Edinburgh, London, Slough and Leeds and also promised to reach 20 locations by the end of the year.
The company said it would initially focus on areas of heavy footfall, where improved data connections and speed are most needed, including London Waterloo and King’s Cross train stations, around Haymarket in Edinburgh, Trinity shopping centre in Leeds and Belfast city centre.
It will also be introduced around sporting venues such as Twickenham and Arsenal’s Emirates Stadium, O2 said.
5G technology is the next generation of mobile network and is expected to offer internet speeds several times faster than that of current-generation 4G.
O2’s launch follows that of fellow operators EE, Vodafone and Three, who each began their own rollouts to towns and cities around the country earlier this year.
Mark Evans, chief executive of O2 parent company Telefonica said: “Today is a significant moment for our customers and our business as we switch on the O2 5G network.
“We’re launching with a range of tariffs that make it easy and fair for customers to access 5G, with flexible plans that cost no more than 4G.
“We’re also switching on 5G in important parts of towns and cities first, places where it will benefit customers and businesses most.
“I believe 5G is going to revolutionise the way people and businesses use mobile connectivity, unlocking huge possibilities for our economy and society.
“No one in the country has all the answers today, but I’m excited about getting it into the hands of our customers and working with leading partners to help shape the future of 5G for the next generation.”
Digital Minister Matt Warman said that the latest network launch of 5G would help to make the UK a leader in the technology industry.
“We want the UK to be a global leader in 5G and for the majority of the population to have access to a 5G signal by 2027,” he said.
“This investment by O2 is a great step forward in rolling out the technology across our home nations, providing world-class infrastructure to help us grow our economy and compete strongly.”