CORRO-OY is the biggest barrio of Santol in terms of population with 1,899 residents as of 1995. It lies in the westernmost part of Santol adjacent of Pagleddegan, Balaoan, La Union with eight sitios namely: Ambagat, Killo, Guilong, Pilaoan, Corro-oy Norte, Ciudad and Corro-oy Proper.

Corro-oy means continues erosion or falling movement of earth, sand or particles of rocks. The river beside the present school continue to show this movement and had it not been for the Maluyo River, accumulated eroded particle would have formed mounds to cover the wide river bed.

Killo got its name from the crooked or curved river from bursting spring among rolling hills and thick forest during ancient times. “Killo” means crooked or curved.

Ambagat derived its name of an ancient practice of the people there which is composed of dancing, praying and singing called “Bagat“, hence, Ambagat. The sitio is now inhabited by Christian Filipinos and the “bagat” vanished.

Pilaoan lies east of Killo. The name was taken from the numerous tiny footprints on the soil filled with glistening water-small ponds of water which we call “pilao”, hence Pilaoan.

Long ago, people go out at night and fish with bamboo torches or kerosene lamps in big decorated dried round apo shells. Visitors saw the numerous lights at night and compared them to city lights, hence, that place north of the Maluyo River was called Ciudad.

The brook at the place meets the Maluyo River with a sound o-ong-ong-guilong-guilong hence Guilong.

Nabsang is a human practice which means deep cut. In this sitio, there is now a cut across a high narrow mountain which old people say was made visitors in order to shorten the way.

The present sit Corro-oy Elementary School was once called Moncayo when Santol was still under Mt. Province. A few meters from the school, the first Roman Catholic Church, St. Isidore, was built by Santol and Balaoan residents under the leadership of Fr. Aurelio Morales, a former parish priest of Balaoan, after the people of the Poblacion gave up the idea.

Corro-oy now boast a barangay hall, a plaza complete with basketball court and a barangay health center, 2 day care centers and a police substation/CVO and a vast network for farm to market roads (earthroads).