You can only register with RHN if you have paid RedHat for a support subscription (sometimes called "license" although that is not actually the right term).
If you have already paid Oracle for an ULN subscription, you should probably use that so you won't waste the money you've already paid.
Oracle apparently allows you to access the latest updates for free; but staying with a defined patch level (which is often required in enterprise environments) requires a paid ULN subscription.
Registering with RHN would cause your system to receive RedHat updates, causing your system to gradually "morph" from Oracle Linux to RedHat Linux as packages get replaced by updates.