The August issue of Dengeki Daioh has announced the OVA for 『Toaru Kagaku no Railgun / とある科学の超電磁砲（レールガン）』.
The episode of the OVA will be released on October 29th, 2010.
Synopsis: (1st season)
Academy City is a highly developed place in terms of technology. It is said to be 20 to 30 years ahead of the rest of the world. 80% of it’s 2.3 million residents are students. The focus of studies here is directed towards esper powers. Misaka Mikoto, one of the top level espers in town, shares a room with Kuroko Shirai, another high level esper who is a member of Judgement, a law enforcing agency composed of students. Both attend to Tokiwadai, a private school reserved for the high-leveled and the rich. Kuroko’s partner at Judgement, Kazari Uiharu, is a low level esper who studies at Sakugawa middle school. Her best friend and classmate there is Ruiko Saten, a level zero (no esper powers). Together, the four encounter several adventures in the exciting scientific town.