Ōka highly resembles his youngest son Recca. Like his son, he possesses spiky black hair, although Oka's hair is kept in a long ponytail, with a mess of hair handing over his forehead. His facial features were older, and more mature than that of Recca's, possessing an unshaven face, as well as including a long scar going down his right eye. In battle, he wears a traditional Hokage ninja uniform. He also carries a ninja sword.
As Resshin, the color of his dragon spirit form is light orange as a contrast to the other dragons who are in orange in color. The most distinctive feature is his spiky hair and fringes which are red. Ōka's voice while in this form is distorted but in a deeper pitch.
In the final episode of the anime, Ōka's image is briefly seen as his dragon spirit form, Resshin which Kagero recognizes him from its distinctive hair and appearance.
Oka was very similar to Recca not only in appearance, but also in personality. Koku even states that the two of them are both idiots, to which they denied, in unison.
As Resshin, he portrayed himself in a different manner however. Resshin showed himself to be a powerful, and vicious dragon who would do what he wanted without fear of repercussions, and only obeyed Recca because he was amazed that Recca would dare give him an order, or on a whim. However in reality, Oka realized Recca was far too weak to hope to defeat Kurei in battle, so he may have simply been trying to save face.
He was also saddened that his first son Kurei and his concubine Reina were forced to endure the suffering that they did with him being unable to make it stop. Oka seemed relieved to know Kurei never blamed him for this. Likewise, he does not seem to resent Recca for acknowledging Shigeo Hanabishi as his first father, due to being raised by him.
Part in the Story
The true form of Ōka's flame is the phoenix, the same form in which he passed on to his oldest son Kurei. The phoenix was stated by Oka to be the rarest form of flame in Hokage history. Even as a full grown adult, Oka had not seen the true form of his fire, and mostly made use of his flames in standard bursts of fire, or used his sword.
When Oka died with regret in his heart, he became the eighth dragon, and took the name of Resshin. Because his flame had an inner shape, he was able to retain his natural power and did not become a useless dragon.
Karyū (7 Flame Dragons)
Ōka is able to use the Flame Dragons, much like his son Recca.
Resshin is the eighth Flame Dragon and is actually Recca & Kurei's father Ōka. His flame can engulf his opponent and either turn their soul as part of Recca's flame or completely disintegrate the soul. This is Recca's strongest move. He uses this on Yanagi when she dies and makes her apart of his flame.