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.
"It is a curious property of research activity that after the problem has been solved the solution seems obvious. This is true not only for those who have not previously been acquainted with the problem, but also for those who have worked over it for years." -Dr. Edwin Land
GMAT Sample Questions to assess your GMAT Quant.
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IMPROVE YOUR READING COMPREHENSION with the ECONOMIST READING COMPREHENSION CHALLENGE:
Here is the first set along with some strategies for approaching this work: http://gmatclub.com/forum/the-economist-reading-comprehension-challenge-151479.html