He is voiced by Toshihiro Shigetsuka.
He was created to be "an older gentleman" and is an enigma to even the creators. Developers consider his moves to be comical compared to the seriousness of his image, especially the move Joker when it is blocked or misses.  Developers considered Oswald a unique character who's filled with aspects that invite wild speculation such as his treasuring “trust” and loathing “the past.”.  They chose to leave the result of his fight with Shen ambiguous, implying that fans would have been disappointed with the outcome. He was originally intended to be the "good-looking glasses" character for the series.
Oswald was originally going to be a playable character in The King of Fighters XII and The King of Fighters XIII, and would team up with Iori Yagami and a newcomer who wore a kimono. This idea was scrapped after Mature was added to the roster. 
Oswald is a man who is said to be a professional hitman who seems to be in retirement. Ash requested for Oswald to join the tournament based on rumors he heard about him. Since neither knew much about the other, the conditions for joining the team was decided in a poker game; the winner would choose to do what they pleased. Despite having a winning hand, Oswald recognizes Ash's skill and accepts defeat - he introduces himself to his teammates while being brought out of retirement.
It is revealed at the end of the tournament that Oswald joins Ash and Shen Woo to obtain money and the drugs named 'Dragon Pills' (which hint that Oswald is suffering from a disease). Ash agrees to give information regarding the drug to him, explaining that it can only be given to him if he defeats the provider's enemy, Shen Woo. As Ash left Oswald and Shen alone by themselves, a battle between the two took place, the winner of which has yet to be revealed; though the confirmed survival of both Shen and Oswald makes either outcome equally plausible. Either way, Oswald was seemingly out of commission or had decided to not participate in the next tournament.
However, by the time of KOF XIV, his apparent disease is somehow retconned with the existence of Ash vanishing in KOF XIII, and thus it seems he no longer needs the Dragon Pills anymore. Oswald then deems himself to be lucky enough to be free of that burden, and more of his personality opens up in the coming tournament.
Oswald is a quiet and very serious man who is willing to do anything in order to achieve his goals, even going as far as to kill. He acts like a professional and is not easily touched by concepts such as morality.
Though in KOF XIV, upon it being revealed that he no longer seeks the Dragon Pills, he expresses signs of relief in that he feels as he is quite the lucky person, which may have been a notable turning point in his life. It is revealed he is extremely observant and perceptive, rarely ever cold and is willing to talk when appropriate; he starts to develop/reveal more standards out of being a good sport for the sake of the gamble and the fight, though he sometimes may state his pragmatism from time to time without trying to make things too personal.
For unrevealed reasons, he knows veterans like Heidern and Takuma particularly well, even Blue Mary's grandfather (and also by proxy is associated with their respective successors). However, he seems to have a personal vendetta against Xanadu and outright states that he must die. Regardless, Oswald is someone who doesn't enjoy much of his own shady past while being a reasonable person.
- Blending with shadows - Oswald is able to blend with shadows. His clothes are not affected by this power.
Oswald fights with his red-back-colored playing cards akin to slashing and throwing them like knives, using a unique fighting style named after Karnöffel; his suit moves (Spade, Diamond, Club and Heart) can all be uniquely chained-together as a focal point of his gameplay. Many fans have noted that Oswald's card-based fighting style is remarkably similar to X-Men's Gambit (this comparison is most obvious between Oswald's JOKER LDM and Gambit's Royal Flush Hyper Combo only in the Marvel vs. Capcom series); only unlike Gambit, Oswald's style revolves more around bladed-striking in close quarters and less around projectile usage. Oswald even has his own super named Royal Flush, but is done differently from Gambit's version.
All of Oswald's moves are officially rendered with playing card symbols (e.g. ♠ = Spade, Q = Queen, ♠♥♦♣ = Four Suits, 10・J・Q・K・A = Royal Flush and so forth). Also, Oswald's Four Suits DM via the C version inflicts a total of 21 hits (a possible reference to the card game, Blackjack), and his attire possesses both black and red in tandem with the two colors of the playing card suits. His strongest DM (either as an LDM or Climax DM), JOKER, does a total of 52-54 hits in KOF XI (via omitting the first hit from the grab, accounts for a 52 card deck), though the revised version in XIV instead does a total of 56 hits and when you remove the inital 2 hits (done by the initial dashing grab and the wall-slam), accounts for the two 2 Jokers as well (despite only a single thrown Joker card being shown at the end).
Gameplay-wise, Oswald is considered one of the top 3 characters in The King of Fighters XI, with many people considering him the third best in the game right behind Gato and Kula Diamond. It should be noted that unlike most characters' standing/crouching weak punches where they usually can get two in at best, Oswald can cheaply land four to five jabs on his foe creating powerful combos due to both the speed and range of the attacks. Much of Oswald focuses on being able to cancel his suit-based special moves into each other in a fluid-and-planned fashion in order to make the most of his damage.
- Joker - The King of Fighters XI
- Joker ~KOF XIV ver.~ - The King of Fighters XIV
- Kimi wa Hero
- The King of Fighters XIII - in the Elisabeth Team's ending; also in some promotional images of the game
- Days of Memories (fourth title) - cameo with Ash