That takes me back. Yes, we could not use the UEK so we tried using the RHCK. But we were still running into issues trying to debug kmods that we weren't running into on CentOS. I guess we weren't in the target market there for Oracle. It left me with a negative impression that maybe is unwarranted, but I've had plenty of other bones to pick with Oracle so I would be very cautious in depending on them for a platform.
See Java as an example. OpenJDK (and IBM/RHEL, ironically) is the only thing keeping that from turning into a bigger support trash fire.
Second, it would likely be possible to run the CentOS kernel on Oracle Linux. My research for Btrfs ran the other way, but I was able to get both the latest Fedora and Oracle UEK kernels working on CentOS with various levels of Btrfs support.
Oracle Linux 6 includes two kernels - the RHCK (Red Hat Compatible Kernel), and the UEK (Unbreakable Enterprise Kernel). Which one were you using?
I think Oracle only commits to userland compatibility with RHEL, but perhaps not kernel drivers under either distribution kernel.