With the increase in smartphones, mobile OS is playing a crucial role in the increase of mobile browsing. Apple iOS, Windows phone, android & blackberry OS are the top mobile OS. There are some other OS like brew, bada from samsung, meego, symbian & others. Nokia recently released Lumia 900 based on latest windows phone OS. This has received good response from the tech community. Apple iOS on iphone & its app store has taken the mobile world by storm. There are thosands of apps & have been downloaded millions of times. Google’s android is giving tough competition to Apple iOS. Android app store has also thousands of apps & gives great opportunity for smartphone users to variety of apps. All the 4 mobile OS are in the race to get a hold on the rapidly increasing mobile market.