I agree with Adam that LSAT passages are longer and tougher than what you see in the OG. However, this seems like a plus to me - at least for those people that are going to score well on the verbal and therefore see the toughest GMAT Reading Comp passages. (Just don't be too upset to miss some questions on these). I also agree with the fact that law passages are not going to be on the GMAT - however, they are boring an quite technical so that could be good to study as well. It depends on what you are looking for - the Official Guides have former GMAT passages and questions that are, on the average, not at the upper levels of difficulty, but they are official.
I am glad to hear the Veritas Reading Comp book recommended (and of course that is written specifically for the GMAT so that can be even more helpful than the LSAT or other nonspecific material).
As to the OG - yes that should be in everyone's study plan. However, there are only so many passages available from GMAC and they seem to hold back some of the tougher ones. So if additional material is needed there are places to go for that...
by the way, here is a link to a discussion that I wrote about using the LSAT to studying for GMAT reading comp.
https://www.beatthegmat.com/lsat-to-stud ... tml#322872