According to an Amazon Japan listing, the 20th volume of the Ao no Exorcist (Blue Exorcist) manga will be released on October 4 and it will also have an OVA. However, the OVA won't be based on the main characters in Katou's manga, instead, it will depict events in Ao no Exrocist: Spy Game spinoff novel that was released last March. That means the OVA will follow Renzou Shima, a student at the exoricst spy school.
Previously, the 19th volume of the manga was released this past April and included a never-before-aired 13th episode of the second season of the TV anime.