I'd use Interoperability to do this Tim
It sounds like you have an application on one platform (AS/400) that needs to send information to OneWorld on another platform (NT/Unix) - have that information processed, and optionally have the results returned.
To do this, I would investigate the use of xmlCallObject (also called "Thinnet") on the OneWorld machine. I'd use a middleware application such as XPi or, preferably, MQ Series, to manage the transaction between the two servers. All of this is documented in the Interoperability Guide - available on the Knowledge Garden.
There are probably shortcuts, possible using API's to directly pass the information into OneWorld - but they would be extremely unstable. By using a message-based middleware, you'd have far more control with regards to failed messages etc.
Alternatively, if this is a simple "fire and forget" you can have the information pass directly xmlCallObkect as a formatted XML document without the need for the middleware...