Oracle says, There is not structural difference between the 2. The only difference is the support policy, where by Oracle Enterprise Linux (OEL) is available as a free download, and RedHat is Available as a Paid download to use in production.

Oracle Enterprise Linux is based on Red Hat Enterprise Linux and is fully compatible—both source and binary—with Red Hat Enterprise Linux as it includes the exact same set of packages at the same version levels with the same source code as the Red Hat distributions. There are approximately 1,000 packages in the distributions. A byte-by-byte comparison of the source code of the two will reveal no difference, the only changes being the removal of trademarks and copyrights.

In order to get Linux support from Oracle, a user does not have to install Oracle Enterprise Linux, if they already have Red Hat Enterprise Linux installed on their systems.

Fully Compatible with Red Hat Enterprise Linux
Oracle synchronizes bug fixes at regular intervals with Red Hat Enterprise Linux to maintain full compatibility. Whenever a new version of an individual package (an errata) gets released by Red Hat, not just as part of an update release, the corresponding package for Oracle Enterprise Linux is made available very quickly, in a matter of hours. If a package has no trademarks and no Oracle specific patches, it will simply be recompiled and reissued for Oracle Enterprise Linux immediately after going through testing.
If a package has trademarks or Oracle Enterprise Linux specific changes, Oracle will examine the source code and compare it against the bug fixes that have been already applied and released as part of Oracle Enterprise Linux. If the Oracle patches are still relevant, then they are reapplied, but if the problems have been fixed in the Red Hat version, whether in the same or in a different way, the Oracle specific patches are dropped and the package recompiled (always checking for trademarks and copyrights issues) and released as part of Oracle Enterprise Linux via the Unbreakable Linux Network (ULN).

References: doc Oracle Unbreakable Linux April 2010. http://www.oracle.com/us/technologies/027614.pdf