@Abdul
I have never understood the design of the diagnostic test in OG13. It is really difficult in my opinion, especially for someone who is just starting out on preparing for the GMAT. All it does is demoralize you, and it is a bit of a shock for most students. A proper diagnostic test should have an increasing level of difficulty of questions from easy to hard, so one can see at what level they are starting to miss questions. I think GMAC has just been lazy in not writing a proper diagnostic test, they have been using the same set of questions since OG 11th edition.
Your score of 16/24 PS and 19/24 in DS is excellent and I would predict that you would score fairly high on the quant section in the GMAT. As far as the real exam is concerned, the high scorers will get a fair number of difficult questions, about 10, but there will be a fair number of medium level questions as well. The OG13 diagnostic test on the other hand is pretty much all difficult questions, so in that sense it is not representative. Once you take the GMATPrep Practice tests(free ones) and the Exam Pack 1 tests, you will have a better sense of the real exam.
As far as the explanations to the OG13 diagnostic tests, you can find my video explanations here:
https://www.gmatquantum.com/og13th/. Please note the diagnostic questions are the same as in OG12.
Cheers,
Dabral