O'Brien was promoted to commander on 22 January 1813 and left "Bacchante" in April, returning to England as a passenger on Watkin Owen Pell's "Thunder". O'Brien was without a command until 1818 but in 1814 published an account of his shipwreck on "Hussar" and his subsequent escape to Trieste. In 1818, after the end of the Napoleonic Wars, he was appointed to command the 20-gun HMS "Slaney". He served on the South America Station and, after Admiral Sir Thomas Hardy was posted to the west, was senior commander on the Atlantic coast of this station. O'Brien held this role during the early part of the Argentine Civil Wars, a time when Buenos Aires was at war with neighbouring provinces. He was successful in persuading the Buenos Aires government not to proceed with its intention to draft British residents into its army and to appropriate their possessions. O'Brien was promoted to captain on 5 March 1821 and left South America in October, returning to Britain on board the frigate "Owen Glendower".