For a great paper on Oracle OLAP running on Exadata, see:
The Executive Overview section of this paper provides an introduction:
This paper describes a performance demonstration of the OLAP Option to the Oracle Database running on an X2-2 Exadata Database Machine half rack. It shows how Oracle OLAP cubes can be used to enhance the performance and analytic content of the data warehouse and business intelligence solutions, supporting a demanding user community with ultrafast query and rich analytic content.
The demonstration represents users of a business intelligence application using SQL to query an Oracle OLAP cube that has been enhanced with a variety of analytic measures. The cube contains data loaded from a fact table with more than 1 billion rows.
Utilizing Exadata features such as Smart Flash Cache, Oracle Database supported a community of 50 concurrent users querying the cube with queries that are typical of those executed from a business intelligence tool such as Oracle Business Intelligence Enterprise Edition.
With each user querying the database non-stop (without waits between queries) with median query times ranged from .03 to .58 seconds, average query times ranged from .26 to 2.32 seconds, and 95 percent of queries returned in 1.5 to 5.5 seconds, depending on the type of query.
Query performance can be attributed to highly optimized data types and Exadata Smart Flash Cache. Cubes are designed for fast access to random data points, using features such as array-based storage, cost-based aggregation, and joined cube scans. Exadata Smart Flash Cache contributes significantly to cube query performance, virtually eliminating IO wait for the high volume, random IO typically seen with cube queries.