Here are some good tips on how to help conserve battery life for the Samsung Galaxy SII. http://support.sprint.com/support/tutorial/Conserv
This should help in most cases.
Android OS is just short for Android Operating System, which is used for your phone to function at all, in order for your phone to pull up all its graphics, in order for your phone to pull up its apps and open those files to display apps or your internet web pages, to pull up your messages to pull up the function to make calls all are because of your operating system which is constantly running and will be the one thing take up the most battery, can not be removed...unless you want your phone to crash.
not to mention the more you use your phone and are playing with it, pulling up different things on your phone, looking through settings the android OS will be higher, not to mention your OS still is going when your phone is in sleep mode, unless the phone is OFF the OS is processing and using battery
That's the title given to all background tasks in the operating system. You can try a factory reset to see if that fixes the problem. I know some of the Over-The-Air updates messed up my phone temporarily. The Galaxy S2 also malfunctions in weak signal areas. There's unfortunately no fix for that. Hardware reverse engineering has shown that a defective design causes RF signals to bleed into other circuits, like those of the soft screen button controller, when the 3G transmitter is at high power. Some symptoms are high battery drain, random button presses, dead buttons, and reboots. Get rid of the phone if it's still in the return period.