<p>We have a client that has Oracle <em>Standard</em>, and a project that would be ten times easier addressed using OLAP. However, Oracle only supports OLAP in the <em>Enterprise</em> version.</p>

<p>Migration to enterprise is <strong>not</strong> possible</p>

<p>I'm thinking of doing some manual simulation of OLAP, creating relational tables to simulate the technology.</p>

<p>Do you know of some other way I could do this? Maybe an open-source tool for OLAP? Any ideas?</p>

I find that it's the schema that causes most of the issues people have with querying a database.  OLAP forces you to either a flat table or a Star/snowflake schema which is easy to query and comparably faster to the source oltp tables.   So if you ETL your source to a flat table or star schema you should get 80% of what you get from OLAP, the 20% being MDX and analytic functions and performance.
Note that you should get a perf boost with a star schema in relational database as well and Oracle probably has analytic functions in PL/SQL anyways.