Hi Andrew
What you're asking for isn't actually too difficult - I've migrated platforms from AS/400 to SQL to Oracle and back again for many, many customers over the past 12 years or so.
OneWorld provides excellent tools to migrate from SQL Server to Oracle - in fact, I architected the very first migration from SQL to Oracle with the Digitial Equipment migration back on B731 !
However, moving the database platform from SQL to Oracle for OneWorld isn't the issue. The big issue is everything else that touches the data OUTSIDE of OneWorld. For example, if you have any 3rd party utilities, or business intelligence, or even FTP Transfers - all of these are going to have to be looked at, rewritten/reimplemented and changed. That is where the biggest risk to your project is going to come from.
Migrating large quantities of data need to be correctly ARCHITECTED. If you look on this board, you'll see many posts about how to move from SQL to Oracle. My view is that you should use OneWorld as much as possible to eliminate any issues with "null" values by having every row pass through the data dictionary. This can be very slow, however, if you run every table sequentially. Instead, you want to run as many in parallel as possible. Secondly, it is possible taking a large table (ie, the F0911) and splitting a single table up into many, many processes - transferring the data can be then architected to run almost as fast as the disks can handle !
Secondly, you want to also consider the effect of migrating the platform to Oracle now, and then going through almost the same testing effort to get to EnterpriseOne 9.0 ? Instead, you might want to perform both the upgrade AND the data conversion at the same time - especially since going from 8.0 to 9.0.1 will incur major database changes anyway. I know others might not recommend this, but the testing effort is going to be huge for both items - and can be reduced if correctly project managed if both are done at the same time.
Feel free to call me and discuss your requirements.