Hi Verytas,
You would almost certainly be fine using the 2018 versions of the Official Guides. While the OG books are great sources for practice questions, they're not designed to teach you Tactics, patterns or the little 'secrets' behind the GMAT - for those, you'll need Course-oriented materials. To that end, you might want to work on the OGs a little later on in your studies.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) Are you currently studying for the GMAT? For how long have you studied and how many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to apply to Business School?
6) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich