You are right - OG is in order of difficulty. However they don't directly indicate the "level" of each question. Also, you may find higher numbered questions which are easier than lowered numbered questions. I think that the OG clearly gets more difficult towards the back but from question to question the order of difficulty feels very loose.
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IMPROVE YOUR READING COMPREHENSION with the ECONOMIST READING COMPREHENSION CHALLENGE:
Here is the first set and some strategies for approaching this work: http://gmatclub.com/forum/the-economist-reading-comprehension-challenge-151479.html