The iPhone 3G is reaching sales levels of around 27,000 units a week in the UK, Mobile Today reports.
While the site suggests highs of 50,000 per week in its first two weeks of sales, despite supply problems reported at the time, the level of sales are seen to be positive 2 months after launch.
The Apple device is believed to be "hovering" between a 4% and 6% share of the handset market, according Mobile's source "close to the matter".
O2 is hoping for a pre-Christmas sales boost for the device when the Pay & Go version - from £350 - goes on sale in the UK from 16 September.
An O2 spokesperson is quoted as saying "We have a strong relationship with Apple and they are pleased with the sales of the iPhone to date".