Ohmu (pronounced Ohm in the film) are giant insects that exist in the Sea of Corruption.
Enormous crustaceans resembling pillbugs, the Ohmu play a very important role in the story of Nausicaä. The Ohmu are greatly feared by many people in Nausicaä's world: though they are ordinarily docile, killing any insect while in the forest will drive any nearby Ohmu into a berserk rage.
Due to their great size, they can destroy entire settlements in this state, and the spores from forest plants which they scatter as they move can kill crops and render an area unsuitable for human life if left unchecked.
However, the molted exoskeletons of the creatures (DE1 pg 9 first appearance) are stronger than ceramics, capable of resisting most human weaponry, and thus highly sought-after as material for weapons, tools, and structures. The Ohmu possess a hive mind with which certain sensitive persons may communicate.
The Ohmu are a unique creation indeed. Their compassion is overwhelming yet at first appearance they seem to be bloodthirsty evil bugs. But the fact of the matter is that they love, and they care for their fellow Ohmu. They care for all living creatures, and they cry when they kill. They have no choice, when their eyes turn red with rage they go on a rampage, even though they don't want to harm anything they still do.
An Ohmu abandons its shell and skin every so often, when injured, they bleed blue blood, and they have 14 eyes. They release spores that land on the ground and grow into miasma-producing plants.
One scene of the manga has (the main character) Nausicaa, her dress dyed blue with Ohmu blood, being held into the air by the feelers of the Ohmu. At first look she looks like a young lost soul, standing in a field of golden grass.