From 15 June, O2's Pay Monthly and Business customers will be able to use their UK plans abroad - in 47 European countries - at no extra cost.
The move echoes the scrapping of roaming charges in similar destinations by EE, Three and Vodafone.
It coincides with the incoming abolition of such fees by the European Union on 15 June.
O2 customers will be able to take their UK plan into some non-EU countries as well.
These include Iceland, Switzerland and Monaco.
When travelling in the Europe Zone outside the UK, O2 customers with the right plans will be able to make calls and send texts to any other country in the zone at no additional cost.
Receiving calls and texts - and using data plans - is also included.
The mobile operator added that customers would not need to take any action to enjoy the benefits of the change.
While the move would certainly benefit some customers, it did not go much beyond what the operator would have to do under the new EU rules, said Kester Mann, a telecoms analyst at CCS Insight.
Brexit time bomb?
Mr Mann added that UK operators might find it difficult to reintroduce roaming fees once the UK left the EU in two years' time.
"I think it would go down very, very badly with customers - it would be a very bold and perhaps foolhardy option," he told the BBC.
"It would be very difficult for them to do that just because the UK is such a competitive market and we've moved such a long way from roaming."
Of course, mobile operators had taken a "financial hit" from not being able to charge roaming fees as they had in the past, Mr Mann said.
Instead, they were increasingly trying to recoup that revenue through other means, he added.
For example, some had created higher end packages offering roaming at no extra cost in even more destinations abroad.