O'Neil then joined 2nd Brigade of Marines under Logan Feland and embarked for Nicaragua, where he participated in the jungle patrols against armed bandits under Augusto César Sandino until July 1929.

He was subsequently ordered back to the United States and joined the Marine Barracks at Naval Air Station Pensacola, Florida.

While aboard of Wyoming, O'Neil was promoted to first lieutenant on November 1, 1933, and participated in the training cruises for midshipmen and NROTC cadets to various destinations, including European ports, the Caribbean, and the Gulf of Mexico.

[1] O'Neil returned to the United States in early 1941 and joined the newly established 6th Defense Battalion under lieutenant colonel Charles I. Murray in San Diego, California.

[4][1] Following the United States entry into World War II, O'Neil participated in the defense of the atoll against possible Japanese attack, which really took place between June 4–7, 1942.

The battalion was later ordered to take part in the New Georgia campaign under Colonel William J. Scheyer in summer 1943 and O'Neil commanded his group during the Landings on Rendova and the Battle of Munda Point.

[4][7][1][8][2][9] O'Neil was ordered back to the United States in September 1944 and joined the headquarters, Marine Corps Base San Diego, California, where he remained under the command of Major general Archie F. Howard for the rest of the War.

O'Neil retired from the active duty on July 1, 1957, after 30 years of commissioned service and was advanced to the rank of Brigadier general for having been specially commended in combat.