ONE is a one-on-one 3D fighting game. The story is about a tournament where fighters enter to come out on top and become "the one". At the start a custom fighter can be designed with options to alter the appearance through gender, face and skin colour, haircut, and outfit (top, bottom, footwear, gloves, accessories). Single-player games can be played in a Story, Survival, Training and Versus mode. The player can fight with only one type of character, but new moves are unlocked gradually by progressing through the Story mode, along with more customization options.

The fighting system with punches, kicks and combos is based on three stances (offensive, neutral and defensive) that can be switched on the fly. The poses determine the movement (quick, medium, slow), types of attack (quick and powerful combos, mixed strikes and some combos, and quick but weak strikes), defense (dodging, blocking and deflecting, and very easy blocking), and grip (one-hand grab for offensive and neutral, two-hand grab for defensive). The offensive pose is also the only one that allows for a counterattack after dodging.

The player's performance through the game is calculated through an ELO level that defines the progression. The Training mode is used to practice, in Survival the player has to defeat as many fighters as possible with one life and limited time, and Versus is one-on-one games against an AI opponent where it is possible to choose the opponent, make it a rated game or not, and define the rounds, time and enemy health. Opponents include bikers, boxers, clubbers, goths, kickboxers, punks, rappers, soldiers and more. Multiplayer against another player is supported through Bluetooth. The player's ELO score can be uploaded using the N-Gage Arena feature to compare rankings worldwide.