@MrViggy

The .NET framework indeed only runs on one platform but you could compile your C# code for multiple platforms via the mono framework but idk if OpenTK can be used with it. It's a good start when trying to learn OpenGL though, but if your looking into actual cross-platform graphics development then I would suggest using freeglut which allows you to easily open windows on multiple platforms and also has several event handlers that you can assign methods to. You can also use straight c++ and OpenGL in it .