O'Callaghan, Sir Francis Langford (1839–1909), civil engineer, was born 22 July 1839 at Drisheen, Co. Cork, second son of James O'Callaghan, JP, and Agnes O'Callaghan (née Langford). Educated privately, he entered QCC, where he studied civil engineering before working on railway construction in Ireland and Wales. He sat the Indian public works department open competition, receiving high grades in the examination, and was appointed probationary assistant engineer on 13 June 1862. Rising rapidly, he became an executive engineer on 1 April 1866, reached the first-class rank in 1871, and had worked his way up to superintending engineer first class by 1886. Involved in the construction of all the major railway lines in India, he was made a companion in the Order of the Indian Empire (CIE) in 1883 for his work constructing the Attock bridge over the Indus river. His exemplary work designing the railway through the Bolan pass to Quetta led to his becoming a companion in the Order of the Star of India (CSI) in 1887.
A fellow of the Royal Geographical Society, he was an associate and then full member of the Institution of Civil Engineers. On 9 May 1889 he was appointed chief engineer first class and was made consulting engineer to the government of India for state railways. In total he received the thanks of the Indian government five times for his work on various railways and bridges. On 8 August 1892 he was appointed secretary to the public works department; he retired two years later. The Colonial Office then gave him responsibility for designing and constructing the Ugandan railway, a task he discharged with his usual tact and ability. For this, and in recognition of his other services, he was made KCMG in 1902.
He died suddenly 14 November 1909 at his home at Clonmeen, Epsom Road, Guildford, Surrey, and was buried at Holy Trinity church, Guildford. He married (22 September 1875) Anna Maria Mary Powell; they had one son, Francis Reginald Powell O'Callaghan (1880–1910).