OD
OD turns up in 2 fields: photonics, mechanical. Here is what it means to the people in each one.
OD in photonics
Optical Density: a logarithmic measure of how strongly a filter or laser eyewear blocks light at a given wavelength.
Optical Density, the base-10 logarithm of the inverse of transmission, so OD 3 passes one thousandth of the light and OD 7 one ten-millionth. Laser safety eyewear and filters are labeled with OD per wavelength range, and the required OD is calculated from the laser's power or energy against the maximum permissible exposure. Neutral density filters for attenuating beams are graded the same way.
"Goggles say OD 7 from 1000 to 1100 but only OD 2 at 532, so they're useless once the doubler is in."
OD in mechanical
The outside diameter of a cylindrical object like a pipe, tube, or shaft.
Outside diameter is a critical dimension for mating parts, such as fitting a bearing over a shaft or sliding a pipe into a fitting. In piping standards, the OD often remains constant for a given nominal size so that threads and flanges match, while the inside diameter changes depending on the wall thickness.
"We need a sleeve that fits over a two-inch OD shaft with a slight interference fit."