At which brain structure do most optic nerve fibers cross to the opposite side?

Most optic nerve fibers cross to the opposite side at the optic chiasm.

The optic chiasm is an X-shaped structure at the base of the brain, near the hypothalamus. Fibers from the nasal half of each retina cross there, while fibers from the temporal half remain on the same side. This arrangement means that each cerebral hemisphere receives visual information from the opposite half of the external visual field.

After the chiasm, the fibers are called optic tracts rather than optic nerves. They mainly project to the lateral geniculate nuclei of the thalamus, which send signals to the primary visual cortex through the optic radiations.

The crossing is important clinically. A mass pressing on the central optic chiasm, such as some pituitary tumors, can damage crossing nasal-retina fibers and produce loss of peripheral visual fields in both eyes, called bitemporal hemianopia.