The prefix OM stands for optical multimode and OS stands for optical single-mode. OM1 and OM2 are older multimode grades. OM1 has a 62.5 micron core, and OM2 has a 50 micron core. OM3 and OM4 also have a 50 micron core but are laser-optimised, which lets them carry higher-speed signals over longer distances than OM1 and OM2. OM4 supports a longer reach than OM3 at the same speed.
OS2 is the standard grade of single-mode fibre for long-distance and campus backbone links. Single-mode fibre has a core of only 8 to 10 microns, so light travels a single path with low loss.
APAR offers multimode cables using OM1, OM2, OM3 and OM4 fibres, and single-mode cables using G.652D, G.655 and G.657 fibres. The right grade depends on the link length, the speed and the equipment already installed.