Optical Singlemode 2, the cabled singlemode class built on low-water-peak fiber of the G.652.D type, with a maximum cabled attenuation of 0.4 dB/km at 1310, 1383 and 1550 nm. The lower OS1 grade allows 1.0 dB/km and is mostly tight-buffered indoor cable. Because the water peak near 1383 nm is suppressed, OS2 supports CWDM channels across the whole band from 1270 to 1610 nm.
Spec just says singlemode, so I'm bidding OS2 loose tube for the campus backbone and OS2 tight buffer in the risers.
Related terms
Singlemode fiber
Optical fiber with a core so small that light travels in a single path, used for long-haul, metro, FTTH and campus backbones.
OM grades
The OM1 to OM5 performance classes for multimode fiber, each rated for bandwidth and reach at a given data rate.
Attenuation coefficient
Fiber loss per unit length, in dB/km, which sets how much a given length of glass will lose.
WDM
Wavelength Division Multiplexing: sending several signals on different wavelengths over the same fiber.