The bulk of the questions in OG12
and OG13 are the same. The main difference is:
1) Integrated Reasoning section in OG13, it gives you online access to 50 IR questions.
2) Quantitative: 81 of the 452 problems are new(the total number of Quant problems are the same)
3) Verbal: 77 of the 455 are new(the total number of Verbal questions is the same)
Essentially, they have dropped 158 problems in Quant and Verbal and replaced it with new ones.
Comprehensive GMAT Quantitative Course: GMATQuantum
Free Video Explanations: OFFICIAL GUIDE GMAT 13, 12, 11, 10; QUANT REVIEW 2nd, 1st.