How to find OS version of iOS device.

NSString *currentOS = [[UIDevice currentDevice] systemVersion];

How to check if current OS is minimum version you need. Suppose minimum you need is 4.2

NSString *currentOS = [[UIDevice currentDevice] systemVersion];

if ([currentOS compare:@"4.2" options:NSNumericSearch] != NSOrderedAscending)

{

     NSLog(@"Yes, it is greater than or equal to 4.2");

}