Symbian will steadily lose share throughout the forecast period as its biggest supporter Nokia transitions its smartphone strategy to Windows Phone. This will present a huge opportunity for competing operating systems to gain footing. Still, Nokia’s commitment to support Symbian devices until 2016 will keep the installed base of Symbian-powered smartphone users on par with its competitors.

Windows Phone 7/Windows Mobile will benefit from Nokia’s support, scope, and breadth within markets where Nokia has historically had a strong presence. Until Nokia begins introducing Windows Phone-powered smartphones in large volumes in 2012, Windows Phone 7/Windows Mobile will only capture a small share of the market as the release of Mango-powered smartphones are not expected to reach the market until late 2011. Nevertheless, assuming that Nokia’s transition to Windows Phone goes smoothly, the OS is expected to defend a number 2 rank and more than 20% share in 2015.

iOS was the third ranked OS going into 2011 and will remain a force in the mobile phone market throughout the forecast. After an initial explosive growth period, iOS is expected to grow at a more modest pace throughout the latter half of the forecast as the smartphone market matures and diversifies. Although a small market share decline is expected, IDC expects significant overall shipment volume growth through the end of 2015.

BlackBerry OS is expected to maintain its position as a Top 4 smartphone operating system over the forecast period. Like iOS, the BlackBerry OS will experience market share decline even as shipment volumes grow throughout our forecast.