O'Callaghan became famous for his identification of a small scrap of papyrus discovered in Cave 7 of Qumran named by the editors as "7Q5". It was stated before his identification that there were no texts of the New Testament found in the caves of the Qumran complex. Claims that the caves of the Dead Sea could hold documents of the early Christian communities were held as naive and even absurd. O'Callaghan did not set out to look for papyri of the New Testament at Qumran, but his interest was identification of the fragments that the editors could not fully clarify. More than a dozen years after certain scraps were printed, it dawned on O'Callaghan that certain of the clear and identifiable letters on them might be part of the occasional New Testament name for the Sea of Galilee, the Sea of Genneseret. Hitherto it had been assumed that such letters were part of the Greek word for "to beget," yet no one had been able to find any piece of ancient literature which fit with this identification of the letters and the surrounding necessary context of other letters. The results of his work with the small fragment brought him to the conclusion that 7Q5 could be a fragment of the Gospel of Mark and he published his investigation in 1972 in his work "¿Papiros neotestamentarios en la cueva 7 de Qumrân?". The reaction of scholars, especially those committed to the conventional wisdom of how the Bible became formulated, was almost universally against O'Callaghan. His identification was viewed as an almost impossible claim since the papyrus itself had been dated prior to the identification as having been written no later than 50 CE, much earlier than scholars thought the New Testament had been written. The Spanish scholar's career was frustrated and he was practically isolated until 1982 when Prof. Carsten Peter Thiede reviewed the research of O'Callaghan. Thiede came to the conclusion that O'Callaghan's proposals were not illogical and his scientific method was serious and possible. Thiede revived the discussion again in his work "The Earliest Gospel Manuscript?" in 1982. However, even today the majority of papyrus scholars disagree with O'Callaghan's conclusion, even though a better theory has yet to be put forth by anyone.
Works
Las tres categorías estéticas de la cultura clásica, Madrid, 1960, 288 pp.
Cartas cristianas griegas del siglo V, Barcelona, 1963, 251 pp.
Studia Papirologica.
Articles
"Carta privada griega del siglo Vp. P.Palau Rib. inv. 7" Studia papyrologica: revista española de papirología 12 : 41–45.
"Nota sobre BGU III 948,13 ." Zeitschrift für Papyrologie und Epigraphik 82 : 103–106.
"La palabra « Camisa » en Los papiros griegos." Aegyptus 76 : 51–55.
"¿Papiro neotestamentarios en la cueva 7 de Qumrán?" Biblica, 1972. Roma. pp. 91–100
"Notas sobre 7Q tomadas en el "Rockefeller Museum" de Jerusalén." Biblica, 1972. Roma. pp. 517–536