Here are links to World War II OOBs, Orders Of Battle. The image above of Table of Organization (TOE) is from World War I and is for illustration purposes.
German Army – numerous different infantry battalions down to gruppe (or plural, gruppen, the German equivalent of the US squad or UK section); URL sent to me by Michael Reese.
Bayonet Strength of four major armies – home page of the above which includes the battalions of the UK, US & USSR.
Numerous National Armies by Niehorster – click on Index button near the top of the page; found on Wikipedia. Wikipedia clarifies the difference between OOB and TOE.
Nafziger – from 1600-1945. Over 6000 orders of battle have been uploaded into the CARL digital library, including the complete U.S. Civil War and World War II collections. The rest of the 3000+ orders of battle are in the process of being added. If you do not find the order of battle you need in the digital library, then you can access the complete collection using the finding aid.
Lists of Orders of Battle for several campaigns and battles on Wikipedia (scroll down to the World War II section).
Handbook on German Military Forces – US Army technical manual from World War II, Lone Sentry version; see CHAPTER II. ORGANIZATION OF THE FIELD FORCES. ibiblio – hyperwar; previous links are web versions. Following are PDF versions: ISCARL; Archive.org, Project Muse, . Amazon appears to have a Kindle and paperback version for sale. I used this a lot when making up scenarios for Tractics.
Handbook on Japanese Military Forces – has a similar list of links to the German edition.
Wargame Campaign 