My dad passed away in December without a will or benefiary and only had a truck that my sister and I would like to donate to charity. Lived in OK.

I'm sorry for your loss.  You can apply for a replacement title from the Oklahoma Department of Motor Vehicles.   Once you have that, you may need legal authority to transfer title to the charity.  If the truck is worth less than $20,000, you can use a "small estate affidavit" for personal property.  This is a legal document in which you or your sister state your father's name, date of death, describe the truck and its value, state that it is the only estate asset, and state that you and your sister are entited to it under the laws of intestacy in Oklahoma.  (Which is true unless you father was married when he died.)  A notary public must witness the signature on the affidavit.  You then file it with the probate court for the town that your father lived in and, after a ten day waiting period, you will have legal authority to sign the title to transfer ownership.  Check out nolo.com for more details about small estate affidavits.  Good luck.