From Discworld & Terry Pratchett Wiki
A rare metal naturally imbued with magic (making it extremely ill-behaved), it is extremely dark in colour and in its unrefined state produces high levels of thaumatic radiation. When used for common purposes it will often yield unexpected results such as the great bell of Unseen University which when struck produces negative sound. The hub of the Discworld is thought to be composed of Octiron giving rise to the strong magical field of the disc.
Compasses on the Disc are made with octiron needles which always point to the strong magical field at the Disc's Hub.
In Lancre, a place where Morris Dancing is taken seriously, the Dark Morris that heralds the change of season from Summer to Winter is danced in (apparent) total silence to the accompaniment of octiron bells strapped to the dancers' bodies.
Octiron is also used by the University for the safe containment and disposal of rogue magic: for instance, the lock on the door of the Octavo's cell was made of octiron, a material capable in theory of staying locked against all magical attacks. Old grimoires and spells with long half-lives are frequently sealed into containers of negatively polarised octiron and sunk in the deepest parts of the Disc's seas, leading to environmental problems such as the awakening of long-extinct fish species, although this could also have something to do with the activities of the History Monks.