In Diablo 4, the term Overpower refers to a game mechanic that is distinct from being Overpowered or OP. While the mechanic itself is called Overpower, it does not mean that it makes your character excessively strong compared to the game's standard balance. Every class in Diablo 4 can interact with the Overpower mechanic in their own unique ways.

When you use a damaging skill, there is a baseline three percent chance to Overpower. You can identify Overpower by the blue or orange damage numbers. Normal damage is indicated by white, critical hits by yellow, Overpower by blue, and critical Overpowers by orange. Do note that damage over time effects, passive abilities, and channeled skills cannot Overpower. This baseline chance to Overpower cannot be modified.

However, Druids, Necromancers, and Barbarians have the ability to guarantee Overpowers under certain conditions. Interestingly, these are also the three classes that have access to fortify, so understanding both Overpower and fortify mechanics is crucial.

To deal significant Overpower damage, you'll need to stack health, fortify value, and Overpower damage multiplier. 

Druids have a natural advantage because their main stat, willpower, not only scales Overpower damage but also increases their overall damage. 

Necromancers and barbarians can still benefit from stacking willpower as it boosts their resource generation and helps unlock rare paragon nodes.

Let's take a closer look at each class's methods of guaranteeing Overpowers:

Necromancers: 

Necromancers have four overall archetypes: bone, blood, shadow, and minion. 

The blood archetype, in particular, focuses on building fortify and abusing Overpower.

The blood core skills, Blood Surge and Blood Plague, have the ability to guarantee Overpowers after hitting enemies a certain number of times. 

The passive skill Rathma's Vigor increases your maximum life and guarantees an Overpowered blood skill after remaining healthy for 15 seconds. 

The defensive skill Blood Mist can be enhanced to reduce its cooldown whenever you Overpower. 

Passives like Tides of Blood and Sacrificing Bone Mages provide additional Overpowered damage multipliers.

Druids: 

Bear druids can guarantee Overpowers with specific conditions. 

The skill Pulverize (enhanced version) builds up stacks as long as you remain healthy, and after accumulating 12 seconds, the next Pulverize will Overpower. 

The skill Provocation guarantees your next skill, regardless of type, to Overpower after staying in Werebear form for 20 seconds. 

The skill Obsidian Slam in the Snake Spirit Boon tree guarantees your next Earth skill to Overpower after killing 10 enemies. 

It's worth noting that druids lack legendary aspects or paragon nodes that directly buff Overpower.

Barbarians: 

Barbarians have two methods to guarantee Overpower, although they are somewhat conditional. 

The first method is by critically striking four times with the basic skill Bash while using a two-handed weapon. This will cause your next core or weapon mastery skill to Overpower. 

The second method is through the Earth Striker's aspect, which guarantees a powerful Overpower after switching weapons 10 times.

Remember that channeled skills cannot Overpower, so permanent Overpowered Whirlwinds are not possible.