There are ten different guns in Oni, including handguns, rifles, rocket launchers, and energy weapons. 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. Since the player can carry only one weapon at a time and ammunition is scarce, hand-to-hand combat is also effective. The player can punch, kick, and throw enemies; progressing into later levels unlocks stronger moves and combos.
There are multiple classes of enemy, each with its own style of unarmed combat. Each class is subdivided into tiers well-ordered with respect to strength. As in Bungie's earlier Marathon titles, tiers are color-coded.
Unlike most console games, Oni does not confine the player to fighting small groups of enemies in small arenas; each area is fully open to explore. The fourteen levels are of various sizes, some large enough to comprise an entire building. Bungie hired an architect to design the buildings. When it was released, one common criticism of the game was that the sparse, Bauhaus design of its levels and its lack of malleable game objects made for a sterile overall feel.
The Oni engine implements a method of interpolation that tweens key frames, smoothing out the animation of complex martial-arts moves. However, frame slippage is a common problem when multiple non-player characters near the player are attacking.
