De link
To publish to a custom domain, you need to add an alias in OpenShift for your application to the domain name you want it hosted at.
$ rhc-ctl-app -c add-alias --alias www.somedomain.com -a <appname>
To complete the process, you need to edit your DNS records and add a cname pointing your domain (e.g. www.somedomain.com) to -.rhcloud.com