Lake Onytavatn is situated at 573,4 m above sea level. Its area is 1,09 kmē, the greatest depth is 23 m, the volume 8,7 Gl, the average depth 8 m, the greatest length 1,9 km and the width 0,9 km.

This lake is the second best fishing lake of the Veidivotn. The catch has been improving since 1994. In 1998 it was 1091. Lake Fremra-Onytavatn nearby has been improving gradually during the last five years. In 1998 the catch counted 153 fishes. The best fishing spots are Fjaran, Klapparnes, Sudurvik, Tanginn, Hofnin og Dypid.