From www.ocparks.com

In 1948, the O'Neills donated 278 acres of canyon bottom land to the County of Orange for park purposes. The O'Neill family donated an additional 120 acres in 1963. In March of 1974, the county purchased 232 acres of land along the park's northern boundary. This acquisition prompted the purchase in 1975 of 30 acres along Live Oak Canyon Road for access onto the new property. On October 5, 1982, 935 acres, known as the Arroyo Trabuco, were dedicated to the County by the Rancho Mission Viejo Company. The Arroyo Trabuco area is now open to public use.

Today, the park encompasses 3,100 acres. Its topography and appearance varies greatly with canyon bottom land, oak woodlands, grassy meadows, and shrub covered hillsides and slopes. At an elevation of 1,000 feet above sea level, the park enjoys warm dry summers and mild winters.