Been doing some testing this WeakAura seems to work. A WeakAura to show the Overpower proc in Berserker Stance. You can also install the Maekor Overpower Alert. WeakAura is more customizable. 
New Aura - Icon
Display
Icon Source - Fallback Icon
Fallback Icon - Search Overpower
Show Cooldown
Glow or not glow
Trigger
Type: Combat Log
Message Prefix: Swing
Message Suffix: Missed
Source Unit: Player
Miss Type: Dodge
Hide-Timed: 5s
Hope this helps.