  The O I 1355{\AA} spectral line is one of the only optically thin lines that
are both routinely observed and thought to be formed in the chromosphere. We
present analysis of a variety of observations of this line with the Interface
Region Imaging Spectrograph (IRIS), and compare it with other IRIS diagnostics
as well as diagnostics of the photospheric magnetic field. We utilize special
deep exposure modes on IRIS and provide an overview of the statistical
properties of this spectral line for several different regions on the Sun. We
analyze the spatio-temporal variations of the line intensity, and find that it
is often significantly enhanced when and where magnetic flux of opposite
polarities cancel. Significant emission occurs in association with
chromospheric spicules. Because of the optically thin nature of the O I line,
the non-thermal broadening can provide insight into unresolved small-scale
motions. We find that the non-thermal broadening is modest, with typical values
of 5-10 km/s, and shows some center-to-limb variation, with a modest increase
towards the limb. The dependence with height of the intensity and line
broadening off-limb is compatible with the line broadening being dominated by
the superposition of Alfv\'en waves on different structures. The non-thermal
broadening shows a modest but significant enhancement above locations that are
in between photospheric magnetic flux concentrations in plage, i.e., where the
magnetic field is likely to be more inclined with respect to the line-of-sight.
Our measurements provide strict constraints on future theoretical models of the
chromosphere.
