Hi Prateekj05,
The differences between any two 'consecutive' versions of the Official Guide (for example, the GMAT2018 and GMAT2019) tend to be relatively small - meaning that most of the questions appear in both versions (approximately 15% of the questions are changed from book to book). Thus, if you have the OG18 and you're looking for another OG Book with lots of 'new' questions, then you should use either the subject-specific books (the Quant-only and Verbal-only books) or an older version of the OG (such as the GMAT2015).
The OG books are great sources for practice questions, but they're not designed to teach you Tactics, patterns or the little 'secrets' behind the GMAT - for those, you'll need Course-oriented materials. Thus, if you're looking to significant improve on a prior GMAT Score or CAT/mock Scores, then you will likely need more than another OG.
GMAT assassins aren't born, they're made,
Rich