This extensive collection of printed primary source documents, titled collectively The War of the Rebellion: a Compilation of the Official Records of the Union and Confederate Armies, and known to historians simply as the OR, is the principal body of primary source materials you are required to consult to complete this assignment, per the assignment instructions.
There are multiple ways to access it. Several online versions of the entire OR are available:
E-book versions of the OR are available through the Hathi Trust Digital Library and Google Books.
The Hathi Trust Digital Library version offers full-text search of each volume. Click on the link below for the catalog record of the full set, select "Full view" next to the volume desired. You may enter search terms (e.g. "19th Indiana" "Nicodemus Hill", et. al.) in the box marked "Search in This Text" in the upper right to search that specific volume only. This may help you target your searches.
The volume that contains a substantial amount of reports, correspondence, etc. pertaining to Antietam is Series I, Volume XIX, Part I. One of the several Hathi Trust Digital Library e-book versions may be accessed here.
Some users may prefer to work with a .pdf version; a .pdf copy of OR, Series I, Volume XIX, Part I is accessible through Google Books here
Print copies are available in the Jefferson Library 3rd Floor Reading Room and the History Department Library in Thayer Hall.