Rancho Trabuco was a 22,184-acre (89.78 km2) Mexican land grant in present-day Orange County, California.
[3] Santiago Argüello was a soldier in the Mexican army, and was commandant at the Presidio of San Diego from 1830 to 1835.
With the cession of California to the United States following the Mexican-American War, the 1848 Treaty of Guadalupe Hidalgo provided that the land grants would be honored.
[7] In 1864, Forster added Rancho Santa Margarita y Las Flores to his holdings.
In 1907, James L. Flood, son of the original owner, made good on his late father's promise and conveyed an undivided half interest to O'Neill, Sr. Four months later, declining health caused O'Neill to deed his interest to his son, Jerome.