The new platform created 70 percent of user growth in the month for its Opera Mini product, the world's most widely used cellphone browser.
On April 13, Apple accepted distribution of Opera's browser for its iPhone after a long review, opening a new and potentially lucrative market it has so far closely guarded.
"A very significant part of iPhone users has at least tried it," Opera co-founder Jon von Tetzchner told Reuters in an interview. "We are also seeing very nice usage."
Opera's browser promises up to six times faster download speeds than Apple's own browser, and to cut data traffic by up to 90 percent. Massive data traffic from iPhones has caused problems for many operators' networks.
Opera said in its monthly mobile Internet report that mobile browser users totaled 58.9 million at end-April, up 6.6 percent in a month.
Opera has increased its lead over the iPhone browser in the last few months, and controls 26.7 percent of the market in May so far, according to Web analytics firm StatCounter.
The iPhone browser and Nokia browser followed with 20.1 percent and 14.6 percent of the market respectively.
Mobile Internet usage has boomed since the 2007 introduction of the iPhone, but Opera said global mobile data traffic through its servers grew only 0.6 percent in April from March.
Von Tetzchner said monthly swings in traffic can be large due to holiday seasons and the number of days in a month.
Wireless operators are keen on raising sales from Internet browsing and the social networking boom as revenue from traditional voice calls is declining, but they are facing increasingly congested networks.
This is helping browsers that package data and send only a small amount through wireless networks, like Opera's does.