I don't remember if RAD 7.5 can target WAS 8.5. With RAD 8.5, you tipically configure a WebSphere Appserver runtime (in Server perspective), and if I'm not wrong you may choose which JDK to use (1.6 / 1.7).
Anyway I don't think you may use Oracle (or OpenJDK) with WebSphere. There are, IMHO, at least two good reasons not do to so.
First, even if the whole thing were working, you would end to have a not officially supported configuration. And that's a pity.
Second, why would you want to change JDK / JRE ? WebSphere is shipped with a JDK that's both certified and optimized for the whole WebSphere enviroment / product: classloaders, ORBs,
Java EE services implementations, web console, resource handling (datasources, queues). Changing the JRE will result in having something that's
simply not WebSphere. So I can't see the point.