This was later published by Pitmans as The Economic Design of Rectangular Reinforced Concrete Sections, a book notable for its clarity and concision of style.

Colquhoun had been resident engineer on the Mersey Tunnel, and became an associate of Lord Beaverbrook, involved in the accelerated construction of aircraft factories.

O'Sullivan was appointed its Chief Engineer, and in this capacity tackled a deep water dock scheme in India, a £10m tunnel scheme in Argentina, and major reconstruction work for the Gas Board at Beckton, as well as embarking on his chef d'œuvre, the design of the Assembly Hall at Bristol in which was built the gigantic Bristol Brabazon aircraft.

His work on this led to a paper The Strengthening of Steel Structures Under Load, for which he was awarded a Telford Premium by the Institution of Civil Engineers.

O'Sullivan then needed a job, and took employment as Resident Engineer responsible for a new power station at Kaduna in northern Nigeria.

From there it moved to Westminster, to an area near Parliament which had become favoured by Victorian engineers promoting canals and railways, and was still popular with the profession.

O'Sullivan settled his firm at 14 Queen Anne's Gate, a fine building erected in 1772 by Charles Townley[6] and enjoying a view over St James's Park and ease of access to the Institution of Civil Engineers in Great George Street.

Though undertaking a wide range of work, it played in particular a significant role in the bridge construction and alteration needed under the Rail Modernisation Plan for the West Coast Main Line electrification, which was carried out in England in the late 1950s to the mid-1960s, and in Scotland in the 1970s.

On the morning train one day he fell into conversation with a girl whom he had seen in church and who was working at a firm of estate agents in Piccadilly.

[11] She was born in Dublin, was in India in early childhood, and was educated in convent schools at Wiesbaden in Germany and Farnborough in Hampshire.

O'Sullivan established a family home in Richmond in outer London, later moving across the county boundary to Long Ditton in Surrey.

He was widely read, with a fine sense for language; and was a keen speaker and writer for the general public, publishing occasionally in the daily press and contributing a number of articles on science and engineering to the Children's Britannica.