The MGMAT guides are currently on the 4th edition, not the 8th edition. The 4th edition references OG12. The 3rd edition references OG11.
If you have the 4th edition and OG11, you can find conversion charts here:
http://www.manhattangmat.com/fourth-ed-info.cfmThis chart was specifically created for people who have the 3rd edition books and OG12, not the other way around, but you can do most of the conversion from these documents - see the charts that list problem numbers in OG11 and the corresponding numbers for the same problems in OG12.
For OG11 problems that were dropped from OG12, those problems aren't included in the conversion charts. I've sent an email to the head office to see whether we do have any charts that will allow you to do that conversion.