Details about ONI
Oni is a Rare Decal.
Decals are the base decoration of your Battle-Car. They can either be animated or static. Some decals like Distortion can override paint finishes. You can equip them inside the customization window in the decal tab.
To get the Oni Decal you can either open a specific crate with a key or trade with other players.
You can search for trade offers with Oni here.