Just finished my OPC questions now. Took me just over an hour to do them all, but as others have said you can do the 5 sections individually and come back to the others later, however when you start a section you must complete it in one go. Some sections only a take a few minutes, whilst others involve a bit of reading and some thought. Nothing too taxing or technical, but I found myself second guessing my own answers and thoughts.
As others have also said, there's not really any prep that you can do as its a test to see what your personality and morals are like.