GameGen-O is the first diffusion transformation model customized for generating open-world video games. By simulating various features of game engines, such as innovative characters, dynamic environments, complex actions, and diverse events, it enables high-quality, open-domain generation. Additionally, it offers interactive controllability, which allows for gameplay simulation. The development of GameGen-O involved extensive data collection and processing from the ground up, including the construction of the first open-world video game dataset (OGameData) and efficient sorting, scoring, filtering, and decoupling of titles through a proprietary data pipeline. This robust and comprehensive OGameData serves as the foundation for the model training process.