title: OAuthAuthorizationServerOptions set in Owin Startup
body: <p>As the title suggests, I need to know if I can set the <strong>OAuthAuthorizationServerOptions</strong> elsewhere.
  Basically I need to set the provider up when I have access to <strong>HttpContext</strong> so I can get the current URL.</p>
  
  <p>I have to pass a different <strong>UserManager</strong> instance to <strong>OAuthAuthorizationServerOptions.Provider</strong> because my users change based on the subdomain.</p>