ONI Together: An Oxygen Not Included Multiplayer Mod (WIP)

Note: This is a work-in-progress project. Not to be confused with onimp/oni_multiplayer.

A new mod that introduces multiplayer functionality to Oxygen Not Included, featuring a custom networking layer and lobby system.

Note: This mod is in the very early pre-pre alpha stage.

Join the Discord

Steam workshop: You can find its workshop page here

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Demo

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Whats done and in progress?

A public trello board exists that tracks whats been done, and what is coming in the next update you can find that here

Found an issue?

Raise it on the issues page, and make a bug report on the discord.

Please at least try to include a video if you can. It makes replicating it so much easier

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Why not just contribute to the old multiplayer mod?

I like the old multiplayer mod — I do — and kudos to the guys that made it. But its implementation is very limited without a lot of extra effort, if not a full rewrite.
On top of this, it hasn't seen activity in over 6 months.

NOTE: as of June 6th 2025, when I started this project.

Initially it was just conceptual, but once I got lobbies and packets set up, I knew I was onto something.

Setup

To get started with building the mod, follow these steps:

1. Install .NET Framework 4.7.2
   Make sure you have .NET Framework 4.7.2 installed on your system.

2. Clone the repository

   git clone https://github.com/Lyraedan/Oxygen_Not_Included_Together.git
   

3. Open the solution
   Open the .sln file in Visual Studio (or your preferred C# IDE).

4. Copy the file Directory.Build.props.default and rename it to Directory.Build.props.user file, then adjust the paths
   Open the .csproj file and find the file under Solution Items. Copy it and rename the copy to Directory.Build.props.user, then open that new file. Change its variable GameLibsFolder to point to your local OxygenNotIncluded_Data/Managed folder, for example:

   <GameLibsFolder>C:\Program Files (x86)\Steam\steamapps\common\OxygenNotIncluded\OxygenNotIncluded_Data\Managed</GameLibsFolder>

   then adjust the variable ModFolder to point at your local dev folder, for example:

   <ModFolder>E:\Documents\Klei\OxygenNotIncluded\mods\dev</ModFolder>

5. (optional) Restore NuGet packages

6. Run dotnet tool restore in packet manager console (Or any other viable console)

7. IF YOU'RE ON LINUX YOU WILL NEED DOTNET 6.0 TO RUN THE PUBLICISER SEE INSTRUCTIONS HERE

8. Build the project
   Once the ManagedPath is correctly set, build the project.
   If everything is configured correctly, the build should succeed. If there are missing reference errors, restart Visual Studio, the solution creates a publicized reference library the first time a build runs and this can confuse the IDE.

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Debugging the Game

To attach a managed debugger (e.g. Visual Studio) directly to a running ONI instance:

1. Find the Unity version

Check your player log at %APPDATA%\..\LocalLow\Klei\Oxygen Not Included\Player.log — the first few lines will show the Unity version (e.g. Unity 2022.3.62f2).

2. Grab the development player binaries

Download that exact Unity version from unity.com/releases and navigate to:

Editor\Data\PlaybackEngines\windowsstandalonesupport\Variations\win64_development_mono\

Copy these three files to ONI's installation folder, renaming WindowsPlayer.exe → OxygenNotIncluded.exe, replacing the originals:

• WindowsPlayer.exe → OxygenNotIncluded.exe
• UnityPlayer.dll
• WinPixEventRuntime.dll

Backup the originals first! Otherwise you'll need to verify game files via Steam to restore them.

3. Enable the debugger wait flag

Open OxygenNotIncluded_Data\boot.config in a text editor and append:

wait-for-managed-debugger=1
player-connection-debug=1

4. Attach Visual Studio

Launch the game via Steam — it will pause at a message box waiting for a debugger. Make sure you have Visual Studio Tools for Unity installed, then in VS go to Debug → Attach Unity Debugger and select the game instance.

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Contributing

Contributions are welcome!
If you have improvements, fixes, or new features, feel free to open a Pull Request.

Please make sure your changes are clear and well-documented where necessary.

AI Notice

Some contributors use AI in their contributions. These models vary from ChatGPT, Gemini etc

Personally I am not against the use of AI, I believe AI is a tool not a solution. If you're a contributor who uses AI thats fine

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License

This project is licensed under the MIT License.
Copyright (c) 2025 - 2026 Luke Rapkin