Ono-i-Lau is a group of islands within a barrier reef system (ono is six in the Fijian languages). There are three central volcanic islands, the uppermost parts of the volcanic edifice rising from the Lau Ridge slightly more than 1000 m below sea level and on which the reef and other islands are built. There are also three clusters of coral limestone islets, Yanuya (50 islets) and Mana (46 islets) on the barrier reef and Niuta (7 tiny islets). A sand cay, Udui, is not counted as one of the six “islands”. The group is in Fiji's Lauarchipelago.
The group forms one of the southernmost of the Lau Islands; it is located at 20.80° South and 178.75° West, and occupies an area of 7.9 square kilometers. It has a maximum elevation of 113 meters. It is 90 kilometers southsouthwest of Vatoa, the nearest largish island; the only land further south in the Lau Group is in the two small Tuvana islands of coral limestone and sand.
There are four villages in the group - Nukuni and Lovoni adjacent and Matokana on Onolevu, the largest island, and Doi on Doi.