Omberg, Mountain forest in Östergötland, Sweden

Omberg is a forested mountain in western Östergötland, 10 km long and 3 km wide, rising 262.8 metres above Lake Vättern at its southern peak, Hjässan. Its western side drops in fault scarps with fifteen water-eroded caves, while the top is mostly coarse granite. The mountain is a horst, fault-bounded on all sides, with sandstone and shale of the Visingsö Group exposed near the lake.

During the migration period around 400-500 AD, three hill fortresses were built on Omberg, with Hjässaborg featuring substantial stone wall construction. These fortifications show that the mountain held strategic importance in ancient times.

The name Omberg comes from the Nordic word 'ama' meaning fog, which describes the mist that often rolls across the slopes and water below. Visitors notice this connection especially on damp mornings when thick fog blankets the landscape.

A 38-kilometer loop trail circles Omberg and is divided into three sections marked with orange signs. Hikers can walk individual sections or the entire route depending on their time and fitness level.

Omberg's name comes from Old Norse "ama", meaning fog or mist, giving "the fog mountain". The 1278–91 form Ammobiærh records this. Local legend also tells of Queen Omma, said to have lived on the mountain; one of its hill forts still bears her name, and Verner von Heidenstam retold the stories in a 1914 novella.