Evening Primrose Oil (EPO), whether taken orally or applied vaginally, is thought to produce prostaglandins¹ , which help soften or “ripen” the cervix for labor. While some small studies suggest that high doses of vaginal EPO might shorten²  the duration of the early stage of labor and help with cervical ripening specifically for someone having their first baby, there are no large studies to support that EPO decreases the length of active labor, or that EPO will induce labor. 

While it’s likely not harmful to use oral or vaginal EPO, it can be a messy process, and you need to use more than one dose for it to be effective. Essentially, the research is just not strong enough to recommend EPO for cervical ripening or to induce labor.