Always On Display (AOD) does not consume noticeable battery & no, it does not cause a "burn-in" issue, as some users keep mentioning. It keeps changing it's position on the display & moreover, takes full advantage of the AMOLED display architecture.
This is my first adventure into Android since I went to iOS back with my 4S. I love my new S7. But one question I have is, I have had my phone for 3 days and my battery seems to be running down faster than I expected. When I look at the battery usage. The Android System is always first at 30% and Android OS second at 20%. Is that normal? It seems really high. I understanding it happening the first day as it is building files. But 3 days later?
This "Android System" & "Android OS" consuming more battery is due to the device not going into Deep Sleep state. Some app is stopping to go into Deep Sleep (other than Always On Display because this does not drain battery).
When you buy a new phone, at least give one week for the battery to settle in. If still battery drain issue, then I would suggest anyone (by now this must have been fixed on your phone, because this is an older post) to check for the pre-installed apps (most of which come from mobile carriers) and disable these unwanted apps. If you don't wish to disable an app, check for the permissions that these apps are using & limit those to that only necessary (storage). Then perform a "Wipe Cache Partition" from Recovery Menu. If still the issue persists after a 24-hour window, then take backup & perform a Factory Reset from Recovery menu. This should fix the issue.
I just got my Note 7 about a week ago and have had Android System and Android OS use 20%+ of my battery each almost every day since I've had it. It is an AT&T model and I've disabled most bloatware and added Nova Launcher. I'm an AT&T sales rep and normally can determine the cause of battery drain but this is stumping me and my coworkers.
Disable the pre-installed apps (if you have not already) that come from mobile carriers (AT&T in your case). Then check for the permissions of the other installed apps. Limit those to only what are required (Storage permissions only). Many of these apps take control over the 'Location' permissions or other unwanted permissions & then this battery drain issue starts.
After checking this, I would recommend to perform a "Wipe Cache Partition" of your device from the Recovery Menu.
And then calibrate the battery of your device (Android OS, to be precise), as recommended on
https://www.androidpit.com/how-to-calibrate-the-battery-on-your-android-device
(Not recommended to do this often. Once in four months or so is fine):
1. Discharge your phone fully until it turns itself off.
2. Turn it on again and let it turn itself off.
3. Plug your phone into a charger and, without turning it on, let it charge until the on-screen or LED indicator says 100 percent.
4. Unplug your charger.
5. Turn your phone on. It's likely that the battery indicator won't say 100 percent, so plug the charger back in (leave your phone on) and continue charging until it says 100 percent on-screen as well.
6. Unplug your phone and restart it. If it doesn't say 100 percent plug the charger back in until it says 100 percent on screen.
7. Repeat this cycle until it says 100 percent (or as close as you think it's going to get) when you start it up without being plugged in.
8. Now, let your battery discharge all the way down to 0 percent and let your phone turn off again.
9. Fully charge the battery one more time without interruption and you should have reset the Android system's battery percentage.