Oxygen Not Included can keep local and Steam-cloud colony files in neighboring but different Documents directories. That makes backup deceptively easy: copying only save_files may miss the actively synchronized colony, while copying only cloud_save_files may omit a local-only game. SaveEditor recommends preserving both trees before changing cloud settings and using timestamps plus the game build/DLC context to decide which copy is authoritative. This is safer than assuming the copy with the most familiar folder name is the newest.
Known Oxygen Not Included save-file structure
The researched filename or folder pattern is Colony save files beneath save_files or the Steam cloud_save_files account directory.. File-extension context: .sav The documented format notes are: Oxygen Not Included distinguishes local save_files from Steam cloud_save_files. Both live below the Klei Documents tree, so the active copy can depend on whether cloud saving is enabled for that colony.
Why an extension is not enough
A save filename can identify a likely container, but it does not prove the internal schema, game version, checksum rules, encryption state, or editability. Save compatibility can depend on game updates and DLC branch. A local and cloud copy with the same colony name can represent different timestamps. SaveEditor therefore separates file identification from Tool compatibility and only labels a game-specific Tool Verified when its exact profile/version evidence passes the runtime gate.
Where the format is stored
PC save locations: Windows: Local: %USERPROFILE%DocumentsKleiOxygenNotIncludedsave_files ; Steam cloud: %USERPROFILE%DocumentsKleiOxygenNotIncludedcloud_save_files<user-id>. Storefront context: Steam, Epic Games Launcher. Do not assume save_files is the active source when Steam cloud is enabled; the user-specific cloud_save_files directory may contain the copy being synchronized. This matters because two releases can use different containers or companion files even when the visible slot name is similar.
Back up before inspecting or editing
Back up both save_files and the relevant cloud_save_files user directory before changing cloud settings or moving a long-running colony. Label the copy with game build/DLC branch. A read-only viewer is lower risk than a writer, but the live save should still be copied first. Cloud context also matters: Steam Cloud and Epic cloud are documented; OneDrive can also synchronize Documents if enabled. Keep the original bytes available so any experiment can be reversed without relying on a cloud service or automatic backup.
What this reference does not claim
This page does not infer an undocumented checksum algorithm, encryption key, compression layer, or cross-store conversion from the file extension. Troubleshooting evidence: If a restored colony is replaced, compare local versus cloud timestamps and resolve Steam/OneDrive synchronization before launching again. Avoid repeatedly overwriting both trees at once. Those questions require a verified parser/profile or a game-specific source, not a generic format guess.
Verification context: Reviewed against current PCGamingWiki Oxygen Not Included save-location/cloud documentation.