O'Callaghan progressed to secondary school in Kanturk and at the age of fifteen, he won a scholarship to the Patrician Academy in Mallow.

Following his graduation in 1926 as the youngest doctor ever produced by RCSI, he joined the Royal Air Force Medical Service.

At university in Dublin, O’Callaghan broadened his sporting experiences by joining the local senior rugby club.

Here he fashioned his own hammer by boring a one-inch hole through a 16 lb shot and filling it with the ball-bearing core of a bicycle pedal.

In 1928, he retained his national title with a throw of 49.53 m (162 ft 6 in), a win that allowed him to represent Ireland at the 1928 Summer Olympics.

On the same day, O’Callaghan's brother, Con, won the shot put and the decathlon and also qualified for the Olympic Games.

This put him in third place behind Ossian Skiöld of Sweden but ahead of Malcolm Nokes, the favourite from Great Britain.

The podium presentation was particularly emotional as it was the first time at an Olympic Games that the Irish tricolour was raised and Amhrán na bhFiann was played.

In the summer of 1930, O’Callaghan took part in a two-day invitation event in Stockholm where Oissian Skoeld was expected to gain revenge on the Irishman for the defeat in Amsterdam.

Shortly before departing on the 9,700-kilometre (6,000 mi) boat and train journey across the Atlantic, O’Callaghan collected a fifth hammer title at the national championships.

The surface of the hammer circle had always been of grass or clay and throwers wore field shoes with steel spikes set into the heel and sole for grip.

He wore his shortest spikes, but found that they caught in the hard gritty slab and impeded his crucial third turn.

While the final of the 400m hurdles was delayed, O’Callaghan hunted down a hacksaw and a file in the groundskeeper's shack and cut off the spikes.

[5][6][7] O’Callaghan remained loyal to the NACAI, a decision which effectively brought an end to his international athletic career.