Since the browser choice screen rollout, Opera downloads have more than tripled in major European countries, such as Belgium, France, Spain, Poland and the UK, said Rolf Assev, Opera's chief strategy officer.
Opera launched a new version of its browser this week, which will help it benefit from Microsoft opening the market.
Shares in Opera reversed their losses after the comments and were up 3.2 percent to 19.50 crowns at 1444 GMT.
On December 16, European Union regulators accepted Microsoft's pledge to give European consumers better access to rival Internet browsers in Windows, ending a long antitrust dispute with the U.S. software maker.
Since the start of the week, Microsoft is offering users the option to select from among 12 browsers, including its own Internet Explorer, Mozilla's Firefox, Apple Inc's Safari and Google Inc's Chrome, on the more than 100 million old and new PCs that use its software. Opera is among the five most popular browsers that initially appear on Microsoft's browser selection screen. To see the other choices, the user must scroll to the right. (http://www.browserchoice.eu).
However, there are no indications, other than a horizontal scroll bar, that there are more choices other than the first five browsers shown and smaller vendors are worried they are too hard to find, said Shawn Hardin, chief executive of Flock, Europe's sixth largest browser.
Frankly, nobody knows there are more than five options. We see this as unfair, Hardin told Reuters.
Microsoft said the screen was in compliance with the European Commission's decision.
(Reporting by Tarmo Virki; editing by David Holmes and Karen Foster)