O2 ran over one billion ads in April, according to comScore, many of which were made up of promotions for the iPhone, which the operator has been keen to push to an online audience.
Rival T-Mobile was the second biggest online advertiser in April, Orange was fifth while Vodafone and 3 were not in the top ten UK advertisers, according to comScore.
O2's bilion-plus ads reached 35.4 million unique online customers and made up 1.9 per cent of all ads on UK websites.
O2 has reported that sales of the iPhone are 'steady' after gaining 286,000 new contracted subscribers in Q1 2009 although some of them were pay-as-you-go tariffs upgrading. And the operator seems set to sustain the ad drive as it continues to enjoy exclusivity for selling the iPhone in the UK, given the number of live campaigns running.
A surprise among the online advertising heavyweights was dieting website Oneweek.co.uk, which ranked third in comScore's top 10 list while Royal Mail Group ranked fourth.
The results (in full below) are due to a new initiative from comScore where it ranks the top advertisers and display ad publishers on a monthly basis.
| Rank | Advertiser | Total display ad views | Share of publisher ad views | Reach in unique users |
| 1 | O2 | 1.1 billion | 1.9% | 35.4m |
| 2 | T-Mobile | 750 million | 1.3% | 29.2m |
| 3 | Oneweek.co.uk | 610m | 1.1% | 28.7m |
| 4 | Royal Mail Group | 575m | 1% | 12.2m |
| 5 | Orange | 558m | 1% | 26.5m |
| 6 | Virgin Media | 545m | 0.9% | 27m |
| 7 | Sky | 527m | 0.9% | 25.8m |
| 8 | BT | 395m | 0.7% | 24.2m |
| 9 | Microsoft | 380m | 0.7% | 22m |
| 10 | Camelot | 322m | 0.6% | 19.6m |
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