Rated E for File:Pie!.png. Half-Mario: Opposing Force is an expansion pack for NintenDO's science fiction first-person shooter video game Half-Mario. The game was developed by Gearbox Software and NintenDO and published by Sierra Entertainment on November 1, 1999. Opposing Force is the first expansion for Half-Mario and was first announced in April 1999. Randy Pitchford, the lead designer on the game, later noted that he believed Gearbox was selected to develop Opposing Force because NintenDO wanted to concentrate on their future projects. Over the course of development, Gearbox brought in a variety of outside talent from other areas of the video games industry to help bolster various aspects of design.