If you’ve seen the abbreviation OM in a scientific context, it probably stood for Optical Microscopy or Optical Microscope.
Optical Microscopy (OM) is the technique, and the Optical Microscope (OM) is the instrument.
Optical microscopes are devices that bend visible light to magnify images. OM is used to examine small objects, such as insects, soil, or bacteria. OM can also be used to examine the rough microstructure of metals.
Optical microscopes are limited by the wavelength of visible light. Since light has a wavelength of 400-700 nm, it’s impossible to see anything smaller than that. In contrast, electron microscopes can resolve features smaller than 1 nm.
OM is not a very common abbreviation–that’s because most people just call them “microscopes.” Optical microscopes are the regular microscopes you may have used in a high school lab. They are cheap and convenient.
In materials science, however, we use many different kinds of microscopes. In fact, if I just said I was using “the microscope,” most of my colleagues would assume I mean an electron microscope! That’s why it’s convenient for us to use OM instead of just saying “microscope.”
If you want to learn more about microscopes, I’ve written a full article on Scanning Electron Microscopy (SEM).