About 600 of the OG11 problems are still in OG12 (but a lot of the problem numbers have changed). There are some articles on our site that allow you to convert the problem numbers:
http://www.manhattangmat.com/gmat-official-guides.cfmStart here and click on the relevant sections for whichever books you have.
Note: this will not help you with the NEW problems that only appear in OG12 - these articles don't contain the chapters with the rephrased DS solutions for the new OG12 problems. If you want those, you'd have to get the new books. Alternatively, you can buy OG11 (that's cheaper than buying a bunch of new strategy guides!).