Hi Necron_immortal,
I think that taking the Diagnostic Test is a good chance to know "where" you are. Also, it gives you an impression of how the GMAT is. When I started with my Preparation, my situation was similar than yours (I had 5 years without reviewing any Math) and my Diagnostic Test was terrible
However, it was useful for me because motivated myself to really study.
In my experience, I started my GMAT Preparation with other books than OGs. I used "The Princeton Review" (Book + CD). I think that this book is great for beginners and also has a Diagnostic Test.
Then, I used Kaplan Higher Score (Book + CD) and, after that, both
OG 10th and
OG 11th. Because I am a non native speaker, for the Verbal Section I practiced from the two
OGs. For the Quantitave Section I recommend you the Exercises of
OG 10th which are harder than
OG 11th.
I hope this helps you

Cheers.