The oral part of the check ride has changed since I took it in 2001. Back then it was basically a free-for-all. The DPE could ask random questions about a wide range of subjects. Some you had to know from memory, others you would look up (know where to look).
Since then the oral has gone more to a scenario based Q&A session. When I took my IR ride a couple years ago the DPE had me plan (in advance) a flight. He then asked a range of questions that covered the flight, threw in changes in weather or communications and then wanted to know how that would impact my conduct of the flight. I suspect that the private oral has gone that direction, as well. The CFIs here can better comment on that.
As to whether the ride (including oral) is more difficult than the written, let me say that they are different. You know what you did to prepare for the written. Study, study, study. For the check ride (including oral) you will fly, fly, fly. Much more fun. Study, too. Know the PTS and what you need to know. And if your CFI has had experience with this DPE before your CFI should be able to prepare you for the oral part. Find out who has used this DPE before and see what they experienced. DPEs don't reinvent the wheel for each applicant. I did that for my IR ride and it was very helpful.
Then, look at Ron's check-ride advice on this board (it's a sticky) and follow it. Remember that as long as you don't hear the DPE say you failed, you haven't. I really enjoyed that final approach (VMC, foggles off) to land at the end of the IR ride. I knew at that point that all I had to do was land that 172 without bending anything and I had it made.
Enjoy.