Hey there!
I've done this a fair few times and the solutions out there really depend on the environment (enterprise / mission critical or development). The BEST way would be the Oracle AS/400 Gateway. Here are some important links in that area:
Allow AS/400 apps to access oracle with the Oracle Access Manager:
Installation Guide for the AS/400 Oracle Access Manager
Allow your Oracle apps to access AS/400 tables and be queried using Oracle:
Oracle Transparent Gateway for DB/2
^^^Those products are fairly expensive but super powerful.^^^
Alternately, here are some more academic approaches to the situation:
Here's a technical comparison of the two technologies... It's a little propagandaish*.
Technical comparisons of Oracle and DB/2
Here's a document written from the opposite point of view - Someone moving from Oracle to DB2. I still find it's useful information:
Leverage your Oracle 10g skills to learn DB2...
And another IBM link that has some really great information all around:
IBM Developer Network Search Results
Hope this helps!
*Yes, I know propagandaish is not a real word.