Hello, Sridevi31. What was written above is accurate. I just want to add that you may see some questions cross-tagged, so you may, as a result, encounter the same question through different searches. It is well known that some questions from older editions of the OG were subsequently turned into GMAT Prep questions, and sometimes, the same CR boldface passage may appear in two different questions, one in the OG with certain lines boldfaced, another in GMAT Prep with a different boldface. It can be confusing, but yes, you can find all the questions from official sources on this site. (Very rarely, I have seen questions tagged as both official and non-official, but one extra question here or there should not hurt.) About the only tag I am aware of that may be problematic is "1000 Series." Some of these were official questions at one time, some have morphed on the Internet over the years to become something different, and still others seem to have come from the LSAT originally. Solver beware.
Good luck with your studies.
- Andrew