

Each class is subdivided into tiers of increasing strength. There are multiple classes of enemy, each with its own style of unarmed combat. Since the player can carry only one weapon at a time and ammunition is scarce, hand-to-hand combat is the most effective and common means of defeating enemies. Power-ups such as "hyposprays", which heal damage, and cloaking devices, which render the player invisible, can be found scattered throughout the levels or on corpses. There are ten different guns in Oni, including handguns, rifles, rocket launchers, and energy weapons. The player can punch, kick, and throw enemies progressing into later levels unlocks stronger moves and combos. Oni is a third-person action game, focused on melee combat mixed with some gunplay. Konoko using a move effective on multiple enemies, the Devil Spin Kick She is partially successful in thwarting the plot, saving a portion of humanity. In the game's climax, Konoko discovers a Syndicate plan to cause the Atmospheric Conversion Centers, air-treatment plants necessary to keep most of the world's population alive, to catastrophically malfunction. The player learns more about her family and origins while battling both the TCTF and its greatest enemy, the equally monolithic criminal organization called the Syndicate. She turns against them as she embarks on a quest of self-discovery.

Soon, she learns her employers have been keeping secrets about her past from her. The player character, code-named Konoko (voiced by Amanda Winn-Lee), full name later given as Mai Hasegawa, begins the game working for the TCTF. The government is totalitarian, telling the populace that what are actually dangerously toxic regions are wilderness preserves, and uses its police forces, the Technological Crimes Task Force (TCTF), to suppress opposition. To solve international economic crises, all nations have combined into a single entity, the World Coalition Government. In the game, Earth is so polluted that little of it remains habitable. The events of Oni take place in or after the year 2032.
