The publishers of OG 11th Edition claims the questions are arranged in order of difficulty. How true that is, I'm not sure.
But I'm pretty sure OG10th Edition does not arrange the questions in order of diffculty. But there is a tool (sorry, I can't remember where it is, but it's somewhere in GMATClub. Someone posted the attachement here a while back) that helps to sort the questions into level of difficulty, question type etc. However, you'll need Microsoft Access to run that tool.