The shounen/seinen manga To Aru Kagaku no Railgun
will be getting an OVA adaptation.
The OVA will be released on October 29th, 2010.
Source: MoonPhase Diary
The shounen/seinen manga "To Aru Kagaku no Railgun" is a side story manga of the shounen light novel series "To Aru Majutsu no Index".
Wiki entry for the "To Aru Majutsu no Index":