The son of a shoemaker, O'Moran left Ireland at a young age for France, where he grew up in Morin-la-Montagne before joining up as a cadet on 15 November 1752 in the Irish brigade de Dillon.
On 3 October 1792 he rose to lieutenant général and in 1793 received command of the camp at Cassel, where he found the armée du Nord defending Flanders against the British and Prussians.
Arriving at his post in April, O'Moran toured the strongholds placed under him (Cassel, Bergues, Dunkirk and Bailleul) to put them into a state of readiness and defence.
On representations from the Ferrières division and suspected of having received English bribes, he was suspended, arrested and imprisoned (as was Richardot) by delegates Le Bas and Duquesnoy in August 1793.
He was brought before the Revolutionary Tribunal in Paris under the Reign of Terror and it condemned him to the guillotine for treason "in opposing plans at the moment of their execution".