That was tricky to get working well, and for a while I hobbled along with gtags... but I do have it working now. And it wasn't that complicated, I was just missing an embarrassing piece of the puzzle.

But yes, basically

    (require 'csharp-mode)
    (require 'omnisharp)
    (setq omnisharp-expected-server-version "1.32.8")
    (add-hook 'csharp-mode-hook 'omnisharp-mode t)

Then omnisharp should be able to install and start a Roslyn server on its own with the corresponding commands.

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)

But figuring this stuff out was only a matter of reading the omnisharp-log buffer and guessing what the errors may be caused by.