Olap4j is an open-source java API for OLAP. It can be used in much the same way as a JDBC driver would be used for relational data sources.
I made some very small changes to the driver source code, which should now make it possible for a variety of client-side tools to retrieve metadata and run MDX queries against Oracle Essbase. Specifically, this update lets Olap4j retrieve data from Essbase via XMLA.
For this to work, Essbase needs to be deployed as a HPS service, which one can do through the EAS console. I will post notes on this in a day or two, with screenshots.
Thank you, Luc Boudreau and Julian Hyde, for letting me post a few lines of code!