I had a RedHat consultant visit us this week, so I asked him about this.
Oracle offers an easy way to register RHEL servers to their ULN, in order to migrate them from RHEL to OEL.
However, RedHat does not currently offer a similar way to register OEL servers to RHN: they apparently consider Oracle still a minor player in the Enterprise Linux field.
So, it looks like you cannot register an OEL server to RHN, at least without first manually replacing the appropriate ULN registration tools with their RedHat counterparts. And even if you could do that, it would eventually cause the server to transform from OEL to RHEL, as OEL packages get replaced with RHEL ones. You would be effectively the first tester of a new unsupported migration strategy. Not recommended for production servers.
Since OEL and RHEL are still very closely related and OEL even guarantees compatibility with the corresponding version of RHEL, there are good chances that it would technically work fine. But the problem is support.
If your Oracle Database licenses specify that the database must be installed on a supported OEL server, then turning the server into a RHEL (or a RHEL/OEL hybrid) would clearly be an unsupported configuration. If you needed support from Oracle, they would be entirely within their rights to say only "Please reproduce your issue with a supported platform, then we'll look into it." Even if the database is supported on RHEL too, you might have problems with the database support, as such a hybrid system is neither a pure RHEL nor a pure OEL, but something different and uncertified.
Since the Oracle Database licences can be the most expensive component of your set-up, and support for production databases is usually a critical requirement, I would recommend registering your OEL-based database servers to ULN and maintaining them as standard OEL servers. Having the database and underlying OS come from the same vendor (Oracle) also means that two vendors won't be able to blame each other if there is a problem.
