The O.R.'s are the "Official Records of the War of the Rebellion," a monumental collection of reports and correspondence of the Union and Confederate armies dealing with battles and other operational matters during the Civil War. This indispensible resource (along with the Navy O.R.'s) has been put online by Cornell University. Below this, I'm putting the link to the main Cornell "Making of America" documents index page, since there are other valuable documents on this incredible website. Scroll down a ways on that page to find the O.R.'s.