The occipital sinus is the smallest of the cranial sinuses.

It is situated in the attached margin of the falx cerebelli

Falx cerebelli

The falx cerebelli is a small triangular process of dura mater, received into the posterior cerebellar notch.Its base is attached, above, to the under and back part of the tentorium cerebelli; its posterior margin, to the lower division of the vertical crest on the inner surface of the occipital...

, and is generally single, but occasionally there are two.

It commences around the margin of the foramen magnum by several small venous channels, one of which joins the terminal part of the transverse sinus; it communicates with the posterior internal vertebral venous plexuses

Internal vertebral venous plexuses

The internal vertebral venous plexuses lie within the vertebral canal in the epidural space, and receive tributaries from the bones and from the medulla spinalis....

 and ends in the confluence of the sinuses.