From the OK Policy Institute:
In 1985, Oklahoma voters approved State Question 588 by a 69.7 percent majority. This created the ad valorem manufacturing exemption. Under Article X, Section 6B of the Oklahoma Constitution, all real and personal property that is necessary for the manufacturing of a product and facilities engaged in research and development, which meet certain requirements, receive a five-year exemption from ad valorem (property) taxes. Since 1985, the range of entities eligible for the exemption has been expanded. Click here to read more.