Openshift cloud provided by RedHat have told us to "set up a CNAME record with your DNS provider" if we need to use a custom domain in PaaS.
What SEO problem will occur when we install WordPress on PaaS?
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TLDR: If your DNS records are setup properly your URL links shouldn't care or know that you've switched server backends and SEO won't be affected.
All RedHat OpenShift plans come with custom domains, which is what you'll need to maintain your SEO rankings.
So long as your file structure stays the same from your previous server (or at least maps the same way) you shouldn't run into problems with SEO as the paths will not have changed.
If you don't choose to use a custom domain however, you will be forced to use OpenShift's subdomaining, and you will lose your link ranking without setting up address forwarding.
Using custom domains with OpenShift: https://www.openshift.com/blogs/custom-url-names-for-your-paas-applications-host-forwarding-and-cnames-the-openshift-way