Data Sufficiency

I've done all DS questions from OG and although I made some mistakes, I found the questions relatively easy (overall). I read that OG11 is sorted by difficulty level, but even questions at the end of the chapter were not so difficult.

On the other side I saw in the Internet some DS questions which are way more difficult than those in OG. So I was wondering what's the similarity between DS in OG and real GMAT? Can anybody who went through OG and real test compare?

thanks!