The ostensible purpose of his mission (apart from the typical intent of pilgrimage and perhaps relic-hunting) was that he might gain further instruction from Jerome on the points raised by the Priscillianists and Origenists. In reality, it would seem that his business was to assist Jerome and others against Pelagius, who, after the synod of Carthage in 411, had been living in Palestine, and finding some acceptance there.
After his arrival. John, bishop of Jerusalem, was induced to summon a synod in June 415 at which Orosius communicated the decisions of Carthage and read several of Augustine's writings against Pelagius. Success, however, was not achieved among Greeks who did not understand Latin, and whose sense of reverence was unshocked by Pelagius's famous question, Et quis est mihi Augustinus? ("Who is Augustine to me?") Orosius succeeded only in obtaining John's consent to send letters and deputies to Pope Innocent I of Rome; and, after having waited long enough to learn the unfavourable decision of the synod of Diospolis (Lydda) in December of the same year, he returned to north Africa, where he is believed to have died. According to Gennadius, he carried with him relics of the protomartyr Stephen from Palestine to Minorca, where they were reported to be useful in attempts to convert members of the Jewish community to Christianity.
His next treatise, Liber apologeticus de arbitrii libertate, was written during his stay in Palestine, and in connection with the controversy which engaged him there. It is a keen but not always fair criticism of the Pelagian position from that of Augustine.
The Historiae adversum paganos was undertaken at the suggestion of Augustine, to whom it is dedicated. Orosius argues that the world has improved since the introduction of Christianity rather than declined as others had argued. In response to those who pointed to contemporary disasters, he simply argues out that previous ones occurring before Christianity were much worse. The work, a universal history of the calamities that have happened to mankind from the fall down to about 417, was the first attempt to write the history of the world as a history of God guiding humanity. Its purpose gave it value in the eyes of the orthodox, and the Hormesta (or Ormesta, Ormista) as it was called, no one knows why, speedily attained a wide popularity. Nearly two hundred manuscripts of it have survived. An abridged, free translation by King Alfred is still extant. Bono Giamboni translated it in Italian language.. A still unpublished 14th century Aragonese translation, made by Domingo de García Martín at the request of Juan Fernández de Heredia, comes from Bono Giamboni's Italian translation. The history of Orosius was translated also into Arabic during the reign of al-Hakam II of Córdoba. It later became one of the sources of Ibn Khaldun in his history.
The sources Orosius used have been investigated by T. de Morner; besides the Old and New Testaments, he appears to have consulted Caesar, Livy, Justin, Tacitus, Suetonius, Florus and a cosmography, attaching also great value to Jerome's translation of the Chronicles of Eusebius.