Have you ever managed to get intellisense working for c#? I try every few months with junk like Roslyn but it never seems to work. It just silently fails to do anything at all.
But yes, basically
(require 'csharp-mode)
(require 'omnisharp)
(setq omnisharp-expected-server-version "1.32.8")
(add-hook 'csharp-mode-hook 'omnisharp-mode t)
The annoying issue that at first prevented this from working was that on OS X I also needed
(require 'exec-path-from-shell)
(exec-path-from-shell-initialize)
(setq omnisharp-server-executable-path "C:\\path\\to\\OmniSharp.exe")