This Olympus o-ring is for the main seal on the rear lid of the underwater housing PT-E05. The housing PT-E05 is for the Olympus E-520 digital Camera.
The o-ring resembling a thick rubber band is designed to establish a watertight seal when going underwater. Manufactured to precision tolerances required to hold a pressure seal on your housing and made of elastomeric material, it provides the means by which a watertight seal is formed.