In autumn 1884, he began his priestly studies, first in Turnhout in Belgium to 1886 and then at Urbana Collegio de Propaganda Fide in Rome for 1892.

Offerdahl was ordained a priest on 22 November 1891 by Cardinal Lucido Maria Parocchi and summer of 1892 he returned to Norway.

Then, he was parish priest in Arendal to June 1924, and as pastor of St. Hallvard's church in Oslo, he took over as the apostolic administrator in 1928.

On 12 March 1925, he had been appointed Chaplain, and in June the same year he was provicar, that is representative of the apostolic vicar, Johannes Olav Smit.

Cardinal Wilhelm van Rossum ordained him as titular bishop of Selje.