Okla. Stat. tit. 19 § 168
Persons who are not county employees, but who are performing substantial and necessary services to the county on a voluntary basis without compensation which have been directed and approved by a county officer, shall enjoy the protection of sovereign immunity of the state to the same extent as a paid employee. Such persons may be reimbursed for expenses incurred during authorized official travel pursuant to the State Travel Reimbursement Act, Section 500.1 et seq. of Title 74 of the Oklahoma Statutes, or for training expenses, provided it is indicated on the claim the person is not a county employee, a description of services performed is entered, and the officer, by his approval of the claim, certifies such services were substantial, necessary and germane to the duties and functions of the county office.
Okla. Stat. tit. 19, § 168