Ambrosio Bernardo O'Higgins y O'Higgins, 1st Marquess of Osorno (c. 1720 – 19 March 1801) born Ambrose Bernard O'Higgins (Ambrós Bearnárd Ó hUiginn, in Irish), was an Irish-Spanish colonial administrator and a member of the O'Higgins family.

He served the Spanish Empire as a captain general (i.e., military governor) of Chile (1788–1796) and as the Viceroy of Peru (1796–1801). He was the father of Chilean independence leader Bernardo O'Higgins.

A member of the O'Higgins family, Ambrose was born at his family's ancestral seat in Ballynary, County Sligo, Ireland. He was the son of Charles O'Higgins and his wife (and kinswoman) Margaret O'Higgins, who were forced off their lands in 1654 by Oliver Cromwell and became tenant farmers at Clondoogan near Summerhill, County Meath c. 1721. Along with other members of his family Ambrose worked in the service of the Rowley-Langford family of Summerhill House.

In 1751, O'Higgins arrived at Cádiz, where he dedicated himself to commerce as an employee of the Butler Trading House. As an Irishman and a Catholic, he was able to emigrate legally to Spanish America in 1756. Once there, and for some time, he was an itinerant trader in Venezuela, New Granada, and Peru, but, being investigated by the Inquisition, he moved to La Plata Colony, in present-day Argentina, where he tried some commercial ventures. From there, O'Higgins proposed to open easy communication between Chile and Mendoza by a way over the Andes, and, his proposition being accepted, he was employed to supervise the project.

About 1760, O'Higgins enrolled in the Spanish Imperial Service as draughtsman and then engineer. He was directly responsible for the establishment of a reliable postal service between La Plata colony and the General Captaincy of Chile. On his first harrowing journey over the Andes mountains separating Argentina and Chile during the winter of 1763–64, O'Higgins conceived the idea of a chain of weatherproof shelters. By 1766, thanks to O'Higgins' efficient execution of this plan, Chile enjoyed all-year overland postal service with Argentina, which had previously been cut off for several months each winter.

In 1764, John Garland, another Irish engineer at the service of Spain who was military governor of Valdivia, convinced him to move to the neighbouring, and less established, colony of Chile as his assistant. He was initially commissioned as a junior subaltern in the Spanish army.

Following the designs of O'Higgins the mountain huts known as Casuchas del Rey were built in the 1760s to secure communications across the Andes.

In 1770 the Governor of Chile appointed him, now in his late forties, captain of a column of cavalry to resist the attacks of the Araucanian Indians, whom he defeated, and recovered big swathes of territory that had been lost by the Spaniards and founded the fort of San Carlos in the south of the province of Arauco. He proved a generous victor, gaining the good-will of the Indians by his humanity and benevolence.^{[citation needed]}