It doesn’t matter if students know what to expect.

An oral exam isn’t the same as reading a written exam out loud. There are a set of learning outcomes defined in the syllabus. The examiner uses the learning outcomes to ask probing questions that start a conversation - emphasis on conversation. A conversation can’t be faked. A simple follow up question can reveal a student only has a shallow understanding of material. Or, the student could remember everything they’ve been told, but fail to make connections between learning outcomes. You can’t cram for an oral exam. You have to digest the course material and think about what things mean in context.

After all, students know what to expect on standardized tests. Some still do better than others :-).