The oral exam would mainly be on the aircraft you are about to use, but be prepared for general knowledge questions as well. My atp oral included questions on nav charts, and the examiner looked at my written test results summary to ask questions on the same topic as any incorrect answer. The written results don't say which question you may have gotten wrong, just the general area it was about. Just check your results and brush up on those topics if you had any wrong.
My atp came while employed using an airline examiner, your results may vary depending on who gives the test.