Mineral rights are a specialized area of the law. In a lot of cases I've had with mineral rights, the cost to transfer title properly exceeded the value. I would do some investigating on your own and try and find out exactly what you have before expending a lot of money on attorneys. Hopefully you'll find that the mineral rights are quite valuable.
You may wish to repost this summary, using an OK address, since most OK lawyers are not subscribed to CA matters. That issue aside, in most cases that I have been involved with, some form of probate action is needed in order to transfer the mineral rights. I have used small estate proceedings where the known value is small or uncertain. Your best bet is to contact an OK probate attorney and find out what rules apply. If you need help finding one, Avvo has a "find a lawyer" link at the bottom of this page. You type in "probate lawyer" and the City or County where the property is located, and you will get a number of lawyers to review.
How can you find out if there are other properties that you do not know about? Lawyers have access to databases that might help you. Otherwise, it can be a very daunting task to locate assets, if the decedent's paperwork is not in order.
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What you have to do to transfer the rights depends on how title is held and the value of the rights. It also depends on how the rest of the estate is titled ( assuming there is something else). You really need to work with a probate / trust attorney. If the decedent died in Ventura County and the trust is a California trust, you need a local attorney first. You may also need an out of state attorney, but it's hard to know without more information. If the assets are in a trust the trustee should be able to direct their transfer with the use of a trustees certification.