First of all, even though the Omen is a "gaming" laptop, you should be running those games while plugged in to the AC.
Batteries have a certain life span and draining them quickly and recharging again will only shorten the life span that much faster. Running games while plugged in to the AC will not hurt the battery.
Overwatch is running low FPS on battery because the system is trying to save on battery power, to allow the laptop to last longer on a charge.
You can change those settings in both the Windows Control Panel/Power Options and set it to High Performance (there are advanced sttings for both plugged in and battery) and in the Nvidia Control Panel.
Its easier if you have the Nvidia Geforce Experience software installed. You can go to Nvidia and download it.
Go to its Preferences/Battery Boost and change the Maximum Frame Rate to its highest setting.
Or go the the Nvidia Control Panel/Manage 3D settings/Power management mode and set it to Prefer Maximum Performance.
Note that changing these settings will drain the battery much faster!