In Open Warfare, the player takes control of an army of worms. The goal of the game is to defeat the opposing army through reducing the health points of enemy worms using various types of weaponry while avoiding friendly fire and other obstacles. The weapons and gadgets the players use in battle include grenades, homing missiles, bazookas, cluster bombs, banana bombs (a Worms staple), dynamite, air strikes, shotguns, and many more. They can also choose from a variety of settings to battle in and make own schemes and teams.
The game is played in rounds with each worm starting with 100 hit points. Each team takes turns controlling one of their worms. The player is usually allocated 60 seconds per turn to move their worm around the battlefield and attack the opponent worms, though this time limit is customisable when playing non-career games. Only one attack is allowed per turn.
In addition to timing individual player turns, each round has a 20-minute time limit. If the round timer runs down to 0:00 before a team claims victory, every worm's HP drops to 1, creating a sudden death scenario. During the course of a round, crates containing health or weapons will occasionally drop and the water level will rise.