Hi,
No, OG 18 won't be useful to invest in. Firstly, you can easily find the 2017 version online (OG 17, Quant/Verbal 17). Secondly, most of the new questions from OG 18/Quant 18/Verbal 18 have already begun to occur here on GMATClub. In my humble opinion, I would invest in the following if you are weak in the following areas:
Verbal- Manhattan Guides or e-GMAT
Quant-
Magoosh, Manhattan Advanced Guide, Jeff Sackmann book or Nishit K Sinha CAT Book. Please note, even though this is a CAT book, the concepts will still carry over to GMAT. Furthermore, this book is advanced as well. If you are struggling with the basics, I would start with the Manhattan Guides first.
Mocks- Manhattan, Veritas, GMATClub Test mainly for quant and most importantly, GMAC's official exams including both of the packs.