  The O I 135.56 nm line is covered by NASA's Interface Region Imaging
Spectrograph (IRIS) small explorer mission which studies how the solar
atmosphere is energized. We here study the formation and diagnostic potential
of this line by means of non-LTE modelling employing both 1D semi-empirical and
3D radiation-Magneto Hydrodynamic (RMHD) models. We study the basic formation
mechanisms and derive a quintessential model atom that incorporates the
essential atomic physics for the formation of the O I 135.56 nm line. This
atomic model has 16 levels and describes recombination cascades through highly
excited levels by effective recombination rates. The ionization balance O I/O
II is set by the hydrogen ionization balance through charge exchange reactions.
The emission in the O I 135.56 nm line is dominated by a recombination cascade
and the line is optically thin. The Doppler shift of the maximum emission
correlates strongly with the vertical velocity in its line forming region,
which is typically located at 1.0 - 1.5 Mm height. The total intensity of the
line emission is correlated with the square of the electron density. Since the
O I 135.56 nm line is optically thin, the width of the emission line is a very
good diagnostic of non-thermal velocities. We conclude that the O I 135.56 nm
line is an excellent probe of the middle chromosphere, and compliments other
powerful chromospheric diagnostics of IRIS such as the Mg II h & k lines and
the C II lines around 133.5 nm.
