From YoYoGames Wiki

The so-called "One Script Games" (OSGs) are small, playable GML scripts that runs a full game by its own. There are two kinds of OSGs.

Object-Filled OSG

Easier to be made, this kind of OSG isn't actually run on the script. Instead, the OSG's code is a script that create objects and sprites with object_add() and sprite_add() GML functions. The game is then run traditionally, with the Game Maker engine doing the step avaliation for each object.

Sequential OSG

This kind of OSG is fairly complex, and therefore more difficult to be programmed. It's often referred as "real OSG", as the game is completelly run whitin the script and no other resource is actually kept on the memory, but the actual script and a room, that is obligatory for any Game Maker game.

Examples of Sequential OSG

The following games are Sequential OSGs:

• LegacyCrono's Plasma(0ne).