OO4O has the most Oracle functionality (object types, associative arrays, etc) , but is also in maintenance mode and may go away as early as 12g.
ADO and OLEDB would be the other main option, but cannot take full advantage of Oracle functionality. You can still use basic things such as clobs and ref cursors though.
The migration path is ODP.NET.
Hope it helps,
You don't need to re-write your existing apps. 11.1 will be supported for some time to come.
I assume you were asking which is best because you're getting ready to write a new app, and in that case I'd discourage against using OO4O for new development, simply because the product is nearing the end of it's lifecycle. As time goes on, OO4O won't be able to use new capabilities in the Oracle DB.
As I understand it, Windows 7 will the last OS to support the runtime as well.