How can I find out what causes "Android OS" to consume battery?
Use an app such as "Better battery stats". This app provides detailed information about all the services running. This is way better than the stock app for diagnostics.

Additionally, a lot of the core services for the system are attributed as part of the "Android OS". There is a very particular bug in Samsung Galaxy S II devices relating to the TouchWiz calendar that keeps doing a lot of unnecessary checks which prevents the device from sleeping properly when the screen is turned off. This causes a huge drain in battery life. Since the TouchWiz on an S II is very similar to the one on a Galaxy Note, you might have the same inherent bug. 

These links might give you a better idea:


StartingAlertService Wakelock...
[FIX] StartingAlertService Wakelock (TouchWizCalendar.apk)