The oral is easy if you know your stuff. The examiner is going to ask you a series of questions on each topic, each getting more in depth, if you get the first two or three correct, you're good, they don't expect you to know everything. What they want to see is when you come to the question that stumps you is that you admit it and DON'T TRY TO BS YOUR WAY THROUGH IT! The correct answer to something you don't know is "I don't know, I would have to look in <name the reference material here> for that." They are assessing your attitude as much as anything. Be willing to admit what you don't know, but know where to look to find the answer.
As far as studying up, study up the chart legend! Examiners love the chart legend lol. They will find symbols on that chart that you never noticed before.
There comes a point in a checkride where the examiner starts instructing, unless they have told you you have failed prior to this, it is a good thing, this is the point in the ride where you have passed in their mind. I got this right after the 'diversion' because I impressed him with a bit of quick chart navigation work. You see, each of those lines of latitude on the chart is 30' apart and a minute of latitude is a nautical mile. Now in a 152 using 90kts for speed and 6gph for fuel burn, that gives me 20 minutes and 2 gallons for each line spacing. When he said "Ok the fog rolled in and it's all IFR in front of us, we need to go East. Can you take us to ABC?" I found ABC on the chart and turned that general direction, then took my forefinger and pinky and placed them on two lines of latitude along a line of longitude to get my spacing, then measured off the distance to ABC and told him "No, we don't have the fuel for that, but I can take US to XYZ along the way and a bit back up to the north." With no plotter or E6B, I came up with the answer he was looking for in about 5 seconds. He even asked "How did you figure that so fast?" Lol. That's when he started instructing. At that point, it's no longer your ride to pass, it's now your ride to fail.