OKLAHOMA CITY (AP) — The number of felony cases filed by Oklahoma prosecutors dropped nearly 30 percent in the year after a voter-backed plan to ease criminal penalties took effect, according to a new study released on Wednesday.

The study, which analyzed felony filings since the new law took effect in July 2017 to make many drug and property crimes misdemeanors, was conducted by Open Justice Oklahoma, a project of the Tulsa-based, liberal think-tank Oklahoma Policy Institute funded largely by the George Kaiser Family Foundation.

The study shows that the number of felony cases with simple drug...