Is omnisharp Server similar to IIS expres

No, the point of the Omnisharp server is not to be a general purpose web server - it's to act as a service. Think of it as an "IDE service" that just happens to exposed itself over HTTP, as that's the easiest way for many clients to talk to a service.

Unless you need to integrate something with Omnisharp yourself (i.e. writing code to be a client), you can probably just think of it as a plugin for whatever you're using. (In most cases there will be an actual plugin, which then talks to Omnisharp.)