OAC documenation: https://docs.oracle.com/en/middleware/bi/analytics-server/datamodel-oas/create-and-manage-level-based-hierarchies.html
Oracle OAC blog: https://blogs.oracle.com/analytics/post/oracle-analytics-best-practices-for-dimensional-hierarchies-level-keys-and-content-levels
OAC best practices: https://blogs.oracle.com/analytics/post/oracle-analytics-best-practices-series-optimal-performance-and-usage
Nasar's Blog 