Oakum – A treated hemp used for caulking joints

Occupant Load – The number of persons for that a building or part thereof has been designed.

Offset – A term used in building when referring to a sunken panel in a wall or a recess of any kind; also a horizontal ledge on a wall formed by a change in the wall thickness at that point. See also “Plumbing Terms”.

O.G. or Ogee – A moulding with a profile in the form of the letter S; having the outline of a reversed curve.

On Center – A term used to define the point from that measurements are taken – from the center of one member to the center of the adjacent member as in the spacing of studs, joists or nails. Also center to center.

Open Stairway – Defined under “Stairway Types”.

Oriel Window – Defined under “Window Types”.

Outlet – Defined under “Electrical Terms”.

Out-of-Plumb – A term used when referring to a member that is not vertical.