Oklahoma's marijuana laws all started almost 90 years ago when, in 1933, cannabis was banned across the state. And it wasn't until 2015 when then-Governor Mary Fallin signed into law a pilot program allowing CBD to be used to treat children with epilepsy. The bill, also known as "Katie and Cayman's Law," allowed the sale of CBD oil with less than 0.3% THC and was the first step towards medical marijuana legalization following a failed attempt to get medical marijuana on the ballot in 2014.
On June 26, 2018, voters approved State Question 788 during the primary election, which legalized medical marijuana and set up regulatory oversight for the sale and licensing of medical marijuana in the state.
Note that the recreational use of marijuana is still forbidden in the state of Oklahoma.