10. C.Oppius, one of the most intimate friends of C. Julius Caesar. Together with Cornelius Balbus, with whose name that of Oppius is usually coupled, he managed most of Caesar's private affairs, and was well acquainted with all his plans and wishes. In the time of A. Gellius (17.9) there was extant a collection of Caesar's letters to Oppius and Balbus, written in a kind of cipher. The regard which Caesar had for Oppius is shown by an anecdote related both by Plutarch (Plut. Caes. 17) and Suetonius (Suet. Jul. 72), who tell us, that when Caesar with his retinue was on one occasion overtaken by a storm and compelled to take refuge in a poor man's hut, which contained only a single chamber, and that hardly large enough for one person, he made Oppius, who was in delicate health, sleep in the hut, while he and the rest of his friends slept in the porch. On the breaking out of the civil war in B. C. 49, the name of Oppius often occurs in Cicero's letters. Oppius and Balbus had frequent correspondence with Cicero, in which they endeavoured to quiet his apprehensions as to Caesar's designs, and used all their efforts to persuade him to espouse the cause of the latter. There is in the collection of Cicero's letters a letter written to him in the joint names of Oppius and Balbus, accompanied by a letter of Caesar's to them, in which the great Roman at the very commencement of the civil war promises to use his victory with moderation, and says that he will try to overcome his enemies by mercy and kindness, a promise which he faithfully kept to the end of his life. (Cic. Att. 9.7; comp. ad Att. 9.13, ad Farn. 2.16, ad Att. 11.17, 18, 12.19.) To the death of Caesar, Oppius continued to hold the same place in his favour and esteem, and in the year before his death we read that Oppius and Balbus had the management and control of all affairs at Rome during the absence of the dictator in Spain, though the government of the city was nominally in the hands of M. Lepidus as magister equitum. (Cic. Fam. 6.8, 19.) After the death of the dictator, Oppius espoused the cause of the young Octavian, and exhorted Cicero to do the same (ad Att. 16.15).

Oppius was the author of several works, which are referred to by the ancient writers, but all of which have perished. The authorship of the histories of the Alexandrine, African, and Spanish wars was a disputed point as early as the time of Suetonius. some assigning them to Oppius and others to Hirtits. (Suet. Cues. 56.) But the similarity in style and diction between the work on the Alexandrine war and the last book of the Commentaries on the Gallic war, leads to the conclusion that the former, at all events, was the work of Hirtius. The book on the African war may have been written by Oppius, to whom it is confidently assigned by Niebuhr, who remarks, "that the work is very instructive and highly trustworthy, but that the language is quite different from that of the work on the Alexandrine war; there is a certain mannerism about it, and it is on the whole less beautiful." (Lectures on Roman History, vol. v. p. 47.) Oppius also wrote the lives of several of the most distinguished Romans. The following are expressly mentioned as his composition: 1. A Life of Scipio Africanus the elder. (Charisius, p. 119, ed. Putschius; Gel. 7.1.) 2. A Life of Cassius. (Charisius, l.c.) 3. A Life of Marius. (Plin. Nat. 11.45. s. 104.) 4. A Life of Pompey, quoted by Plutarch (Pomp.10), who observes, "that when Oppius is speaking of the enemies or friends of Caesar, it is necessary to be very cautions in believing what he says." 5. Probably a Life of Caesar, from which Suetonius and Plutarch appear to have derived some of their statements. (Comp. Suet. Cues. 53; Plut. Caes.17.) After Caesar's death, Oppius wrote a book to prove that Caesarion was not the son of Julius Caesar by Cleopatra, as the latter pretended. (Suet. Caes. 52. Comp. Vossius, De Historicis Latinis, i, 13, pp. 67, 68, Lugd. Bat. 1651.)