The "Original Game" or "OG" refers to Leviathans Core Box Set and related materials, released in the early 2010s.
In 2022, the Leviathans: The Great War Kickstarter Campaign introduced the Leviathans:The Great War update, which is backwards compatible with the Original Game. However, some rules and some details of canon, as well as the year of the initial setting, have changed. In those cases, the "LTGW" content takes precedence.