Verbal OG section is very similar to GMAT verbal. OG quant is easier to the actual GMAT although the last 60DS will be overly difficult. If you try to join of the the GMATclub challenges, you will find the latter also much more difficult than GMAT quant. It all depends on what quant level you are at now. If you are a quant jock, OG problems are way too easy for you. If you are more on the litterary side, then I would still suggest you to go through the whole OG to get used to the different problem types you may encounted in the exam. Kaplan quant problems are very good but under timed condition, on their CATs, they will seem very hard due to their calculation intensive form. I found PR2005 CAT problems fairly representative of actual GMAT problems. You might consider buying it.