first, let me "break" a myth: OG questions as well as gmat questions are not divided into easy/medium/hard classification. this is a simplification used by prep material. the truth is that each gmat question is assigned a difficulty coefficient on a certain continuous scale. it is closer to have 10-20 "levels" of difficulty rather than 3. if indeed the OG is ordered by this difficulty factor - i don't know... but if it is - why not trying go over the questions from the beginning until they are too difficult for you?
also - it is important to remember that what may be difficult to one person may be easy for another. this is not a linear scale. so why not practice the "difficult" questions as well? you might find that some of them are doable...