The default column name for the date inside the NodeInstanceLog was using "date", which is not allowed in Oracle DB.
I've changed the default column name to "log_date" and tested persistence with Oracle DB and it seems to working correctly here.
https://github.com/krisv/jbpm/blob/master/jbpm-bam/src/main/resources/AuditLog.hbm.xml
Let me know if you still have issues.
Kris