Is CentOS in fact based on RedHat's Application Server?
Will Open-Xchange 5 (commercial version) work on CentOS as simply as it's purported to work on RedHat?
CentOS is binary compatible with RHEL. It is compiled from RH's available source code. The only difference is that copyrighted RH logo's etc have been replaced with CentOS logo's.
Your application should run on CentOS in exactly the same manner that it would run on RHEL. The only difference is that the vendor will likely not officially support your installation (if you tell them you're running on CentOS rather than RHEL).
Hope that helps.