your MX record has nothing to do with OWA
Your MX record should always point to your SMTP server.
You just need to setup an A record in your DNS to point to your webserver as usual
If you wish, you can put a redirect HTML page as the default page to route them to /exchange folder
I believe you could also setup host headers if you already have a website for mycompany.com so that mail.mycompany.com gets pointed to a different site on the same IIS box.