The Oni represents repressed anger or wild instincts. It is a terrifying obstacle that must be confronted. Often, the Oni is a part of the self that has been demonized.
Spiritual Significance
They are guardians of hell, but also protectors if tamed. Spiritually, they represent the "Shadow Energy" that can be converted into power if you stop running from it.
Psychological Perspective
The shadow is "Rage." It represents the destructive power of suppressed emotions exploding.