The list of available database dialects in the User Guide at address http://docs.jboss.org/hibernate/orm/5.4/userguide/html_single/Hibernate_User_Guide.html#database-dialect does not list all the available dialects for Oracle.

The User Guide mentions Oracle 8i, 9i and 10g. In the code, there are Oracle*Dialect classes available for 8i, 9, 9i, 10g and 12c. So the User Guide does not mention 9 and 12c at all.