Red Hat 6 has been there a while, so what about certification with Oracle and when? Nothing yet on the Oracle support site, no press releases (maybe I missed one..). But Red Hat had a blog-post about it a while ago (august 2011):
We’re pleased to announce that on Tuesday, August 9, we formally submitted to Oracle full certification test results of the Oracle 11gR2 database (Single Instance and RAC (including ASM) for x86 and x86-64) on Red Hat Enterprise Linux 6. Oracle database certification is a self-certification program whereby operating system vendors perform extensive testing and submit the results to Oracle for audit and approval.