The origin of the Ounalom Pagoda can be traced to as far back as the 15th Century. It was built in 1422 by King Ponyea Yat, the last king of the Khmer empire. It is one of the five original monasteries in Phnom Penh that King Ponhea Yat had built.
Wat Ounalom is recognized as the ‘headquarters’ of Cambodian Buddhism. The patriarch, the chief abbot of Cambodia, resides here. The compound, forty four buildings in all, is about 250 metres north of the Royal Palace along Sisowath Quay at the river front. This monastery was home to over 500 monks and housed a huge library consisting of over 30,000 titles. During the regime of the Khmer Rouge, many of the buildings along with many religious statues and symbols were damaged, but most of which have since been restored. The buildings are used for a variety of purposes. There is the temple itself, schools, libraries, living quarters and the stupa that gives the temple its name; the stupa holds an Ounalum, a hair from the eyebrow of the Buddha. source: http://www.tourismcambodia.org/