Enter the game and report an error

The issue is Steam thinks you have Spaced Out content installed but the files are missing.

If you own Spaced Out make sure the DLC is installed:

1. Right click Oxygen Not Included in Steam
2. Click Properties -> DLC
3. Check the box beside Oxygen Not Included - Spaced Out!

If you don't own Spaced Out follow these steps:

1. Open Documents\Klei\OxygenNotIncluded\kplayerprefs.yaml in Notepad
2. Search for "EXPANSION1_ID.ENABLED: 1" and change the number from 1 to 0.
3. Save the file and start the game.

 In the next update we have added some code to better handle this situation.