To add to what gsalem says, OEL is binary compatible with RHEL, so any RHEL applications will run unmodified in OEL. One of the benefits of Exadata is that it's an appliance product, and everyone uses the same operating system (or one of two now that Solaris is available). This means that configurations and patches are tested by Oracle development using the exact same kernel and operating system as every user in the field.
Addition to marc's comments:
Whilst it is technically possible to use redhat linux, exadata is preinstalled with either oracle Linux or solaris on the database layer, and with oracle Linux at the storage layer.