http://en.wikibooks.org/wiki/Java_Persistence/Identity_and_Sequencing:

"Identity sequencing uses special IDENTITY columns in the database to allow the database to automatically assign an id to the object when its row is inserted. Identity columns are supported in many databases, such as MySQL, DB2, SQL Server, Sybase, and PostgreSQL. Oracle does not support IDENTITY columns but its is possible to simulate them using sequence objects and triggers"

Mark in source code that your solution will not work with Oracle from the box! There were plans to support Oracle in future