Omnisharp in VS Code produces a lot of warnings about a Unity project. Why?

This issue has also been solved in 1.2.0. Just update your vs code package to this version via the package manager. Then regenerate your project files by navigating to Edit -> Preferences -> Regenerate Project Files. Reopening a .cs file should then solve your problem.

This warnings is a known Visual Studio Code Editor 1.1.4 bug, you can ignore it. For me, downgrading to version 1.1.3 of the package fixed the problems.

If you downgrade to 1.1.3 and right after upgrade to 1.2.0 your problem should be fixed!