Your parent's should visit an experienced estate planning to determine their best course of action. You cannot really do anything, it is they who need to act. They have to first determine if they are subject to State and/or federal estate tax and determine what tax saving measures they can take in their estate planning (this is usually important to do before either spouse passes away). If one or both of them has passed away there is much less you can do to save estate taxes. From my cursory research OK currently has no state estate tax or state inheritance tax. If your parents are residents of OK they may be subject to Federal Estate Tax but only if they exceed the federal exemption amounts - currently about $10 million per couple, but set to change after this year. Again, have them visit an attorney for qualified advice to fit their situation.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/