Up to 9i you had to use Tom Kyte's Stragg function to get the children of the level to a single string. From 10GR1 you can use the collect function.
I wrote an entry on my ORACLE101 blog (http://oracle101.blogspot.com/2008/08/oracle-collect-function.html) which can be used in the same way you use the STRAGG function in OBIEE (see: http://obiee101.blogspot.com/2008/08/obiee-children-of-level-converting-rows.html)
Till next time
