But you cannot upgrade OEL5 to OEL6 without reinstall.
It takes some resources ( time and money) to migrate all the databases (wa have 6 production RAC databases). When we use OEL6 in virtual environment we do not need to migrate or reinstall until 2021Q1.
When we use OEL5, at 2017Q1 we need to reinstall OS ( and database software) and migrate all databases ( and reconfigure all applications) to be supported.
I've raised SR with question certified vs. supported, in case of negative answer the only way seems to be the OEL5 way.