One shot developer A developer used once and then discarded.

Opaque:  Describes 1.)  any substance or surface that will not allow light to pass; 2.)  a paint used to block out portions of a negative so that they will not allow light to pass during printing.

Open up:  To increase the size of a lens aperture.  Opposite:  stop down.

Orthochromatic:  Film that is sensitive to blue and green but not to red wavelengths of the visible spectrum.  Abbreviated ortho.

OverdevelopTo give more than the normal amount of development.

Overexpose To give more than normal exposure to film or paper.  The resulting silver density is often too great for best results.

Oxidation:  Loss of chemical activity due to contact with oxygen in the air.