The rig is identical to a Finn comprising a single sail set on a rotating, un-stayed, bending mast.

OKs are built in plywood, G.R.P and composite construction; all forms enjoy equal racing success.

Consequently, every OK develops to suit the owner's style of sailing, while the shape of the hull is defined by a comprehensive set of strict one-design rules ensuring a long competitive life span.

Old boats often only need a rig update and minor constructional modifications to make them competitive, provided they meet modern buoyancy requirements.

In the 60s and 70s, the OK class enjoyed an explosive success, with the total number of boats exceeding 10,000, and large racing fleets building up.