OS2 is single-mode fiber (9 µm core): multi-kilometer reach, it carries all current and future speeds (10G, 100G, 400G, DWDM). It is the standard for cross-connects and room-to-room or building-to-building links — in a data center, when in doubt, choose OS2.
OM4 (and OM5) is multimode fiber (50 µm core): its short-reach optics (SR) are cheaper, but reach collapses as speed rises — 550 m at 10G, ~100 m at 100G-SR4, and 400G gets acrobatic. It remains relevant inside a room, between servers and switches.
With single-mode optics prices having collapsed, the 2026 trend is clear: OS2 everywhere except very short runs. And whatever the fiber: LC connectors, impeccable cleanliness (inspection before plugging avoids half the tickets) and documented patching.