When they released OG12, most of the changes to the Quant section were in the DS chapter - it's not only longer than before, but it's much more challenging. I consider it to be considerably more difficult, overall, than the PS section. It is great practice material, and the newly added questions do feel more like current GMAT questions (even if some of them are actually from old paper tests), but you'd normally expect to do worse in the DS chapter of OG12 than in the PS chapter, just because the questions are harder.

If you're aiming for a 700+ score, you'd probably like to be getting more than half the questions right in the DS section, but you don't need to be at all close to perfect on high level questions to get a good score. Take those questions you find difficult as a learning opportunity, particularly those questions which test important concepts (some questions in OG12 are so idiosyncratic that they almost aren't worth studying - the question about Carl's account balances, for example - as you aren't likely to see anything similar on test day, while others test fundamental concepts like ratios or number properties that are certain to be heavily tested on your real GMAT). If DS is more difficult for you than PS (that's true for almost all test takers), then you know what you can focus on between now and test day to make the biggest improvement.