How are you planning on connecting to the Oracle DB? You can't just connect your existing application to Oracle because the DB won't know what to do with the code. What is the motivation behind the switch? It sounds like a really bad idea.
There is an OpenEdge product called DataServer that allows an ABL client to connect to an Oracle or SQL Server back end. I have never bought it or used it so I can't advise you further. But my understanding from hearing others describe it is that implementing it is not trivial or transparent to your application and you may have to write back-end-specific code to get queries to perform acceptably.
You should research it and conduct a cost-benefit analysis, including first-hand information on costs and potential pitfalls, before committing to changing your back end.