O'Shaughnessy studied forensic toxicology and chemistry in Scotland, and graduated in 1829 with an MD from the University of Edinburgh Medical School.

[5][6][7][8] O'Shaughnessy failed to obtain a position Professor of Medical Jurisprudence in the University of London and his first marriage forced him to join the service of the British East India Company on 8 August 1833 and he moved to Calcutta, remaining in India and serving initially at Gyah and Cuttack before joining the 72nd Bengal Native Infantry.

He joined the 10th Regiment Bengal Light Cavalry in 1835 and was an assistant to the Opium Agent in Bihar till 5 August 1835 when he became Professor of the medical college in Calcutta.

As chemical examiner he developed methods for forensic studies for detecting arsenic poisoning as well as botanical toxins (such as from Plumbago used as an abortifacient[9]).

[10] He went on furlough to England in 1841 to 1844 and returned to join as Chemical Examiner and additionally as Deputy Assay Master of the Calcutta Mint.

[11] His first stint in India was marked by work in the subjects of botanical pharmacology, chemistry, galvanic electricity, and underwater conduction, among others.

He also developed a silver chloride electrode[10] and experimented on adding colour tones to the Daguerre photographic process.

O'Shaughnessy established his reputation by successfully relieving the pain of rheumatism and stilling the convulsions of an infant with cannabis.

[14][15] In 1839, O'Shaughnessy conducted experiments on an experimental telegraphy system that he set up in the Botanical Garden at Calcutta with the assistance of Nathaniel Wallich.

The Candidature for Election to Fellow of the Royal Society stated that he was "distinguished for his acquaintance with the science of Medicine and Chemistry, eminent as a Physician and as a promoter of education among the natives of Bengal".

During the following years O'Shaughnessy wrote on telegraphy-related subjects, including a book of private codes for encrypted telegraphy.

In 1860, O'Shaughnessy returned to Europe for sick leave where he remained in obscurity until his death from senile asthenia at Southsea on 8 January 1889.