I think the OG is good if you know what you're trying to do with the questions. If you're practicing in the first 100-150, you should be trying to go very fast, with no mistakes. This helps you practice easy level questions without making dumb mistakes. If you don't make silly mistakes and the easy ones just bore you, you should only do the final 50 of each section. Those are the only semi-difficult ones in OG.
If you are really looking for harder questions, I recommend Kaplan 800
. It has some really good questions to test you.
Other than, you should do the problems posted on the forums for this site. If you can solve some of the Quant problems posted here, and explain your answers so that others can understand, then you'll be fine come test day.