Om1 has 62.5um (micron) core diameter
Om2 has 50 um (micron) core diameter
It's basically the way the light travels more "efficiently" with the lasers/leds. Smaller core diameter eliminates modal dispersion and enables tremendous transmission capacity over very long
distances. excellent pdf by bicsi
You can get up to 10 times the speed in 50 micron VS 62.5 just because of the effectiveness.
The Differences Although 50-micron fiber features a smaller core, which is the light-carrying portion of the fiber, both 62.5- and 50-micron cable feature the same glass cladding diameter of 125 microns. You can use both in the same types of networks, although 50-micron cable is recommended for premise applications: backbone, horizontal, and intrabuilding connections, and should be considered especially for any new construction and installations. Both types can use either LED or laser light sources. The main difference between 50-micron and 62.5-micron cable is in bandwidth-50-micron cable features three times the bandwidth of standard 62.5-micron cable, particularly at 850 nm. The 850-nm wavelength is becoming more important as lasers are being used more frequently as a light source. Other differences are distance and speed. 50-micron cable provides longer link lengths and/or higher speeds in the 850-nm wavelength.
Side note... If you ever patch 50 and 62.5 together it's kind of like putting two diffrent water pipes together. The data will work pretty good from 50 to the 62.5 but 62.5 to 50 it will have lots of loss. You can patch them to a gbic or adaptor to do the conversion properly. We sometimes use a media converter that looks similar to the fiber to ethernet type but have 2 gbics instead of a gbic and rj45. The gbics work properly with both diameters of fiber.
Reference for most of this link