Hi Wip,
I'm not 'technically' qualified to provide the requested info regarding Oracle Database Manager or Oracle Transparent Gateway. Though, to my knowledge, they are only Data Gateways - and don't really allow you to interface 'COBOL' or other Applications (I may be wrong) - they allow you access Date across dissimilar systems.
EnterpriseOne is, for the most part, Database Ignorant. You can actually split your EnterpriseOne data across different Databases (usually, Oracle, DB2 or SQL) and 'usually' only one Database at an instance. You can Map your E1 Data across more than one database and separate databases - if your organization wants to apply the effort....
Thus - from the E1 perspective: You can run E1 and the Oracle DB on AIX and OCM/Map tables into your World Instance - making that info on your iSeries available to E1. Your team 'should' understand the concept of a Virtual Table (you create a File Description in E1, do not Generate, then OCM to that file on the remote system - it may require you to create an SQL View/Logical on the World side).
If you want World to see data on your E1 system - the Gateway(s) are the way to go.
In my earlier days - we did use the Oracle Gateway between an AS/400 and a DEC Alpha Oracle Database. It Worked GREAT! We could easily read/write data across/between E911 and Other Law Enforcement systems.
If you are looking for the ability to run applications between systems - that may be a little trickier. That ability was not available ten years ago and we created some down-right trickory. On both systems we created an 'Application' table. We would write the name of the application we wanted to run on the 'other' system, and a process would read the table and run that process (with any passed variables/options). I believe that gateways have gone a long way since then, but I am unaware of current processes.
It has been a significant time since I've played the Gateway Game - Let me know if you need more.... Feel free to call, if you think I would be of any assistance.
(db)