Hi inquizzitive,
The OG books are great sources for practice questions, but any book that you use will be 'limited' in some way (no book can completely prepare you for every variation of a question that could occur on the Official GMAT).
In that same way, doing practice problems out of a book is NOT the same as facing a FULL 4-hour computer-adaptive Exam on a computer.
The major concepts, rules, patterns, question-types, etc. that appear in the OG will appear on your Official GMAT, but the adaptive nature of the Test means that you will likely see questions that have a 'twist' to them. You'll likely know the 'rules' involved, but you'll have to do a bit more work to get to the solution.
GMAT assassins aren't born, they're made,
Rich