Ogilvie's book is a line-by-line commentary of Livy's monumental History of Rome, published in 142 volumes during the time of Emperor Augustus. Even though Livy's huge work is partially lost (only volumes 1 to 10, and 21 to 45 survive), it is our main source for the Roman Republic alongside Cicero.
Ogilvie only deals with books 1-5, in which Livy tells about the mythological events before the creation of Rome, the Roman monarchy, the foundation of the Republic, and the years down to the Sack of Rome by the Gauls in 390 BC. Most of these events are mythical; while the more recent ones are built on traditions, they have been so corrupted by the years that they barely reflect historical truth. As a result, it is imperative for every student of Rome to read the commentary written by Ogilvie to separate truth from fiction in Livy's text, as well as to decipher many other obscure reflections he made.
Ogilvie's Commentary became very influential when first published in 1965. It encouraged many historians to explore the early years of Rome (such as Tim Cornell). Several authors have also followed his methodology, notably John Briscoe (who published the commentaries for Livy's books 31-45), and Stephen Oakley (for books 6-10). However, the latter commentaries are much more developed (especially Oakley's) and Ogilvie's have aged a bit. He does not comment on many words or sentences in Livy's text. It can be seen from page number alone, while Ogilvie took 788 pages for the first five books, Oakley wrote on 758 pages for book 9.