Thank you for your post.
I just want to confirm that the Oracle 11G driver exists on the server where the file is being hosted.
Also, go to another computer that does not have the driver installed, launch FileMaker Pro, and attempt to connect to the hosted file. Does this connect successfully?
When do you receive the error message? That is, are you able to open the file in FileMaker Go 12 but then fail when you attempt a certain task?