  The aim of this paper is to understand the formation of the OI line at 7772
\AA\ in the solar chromosphere. We used SST/CRISP observations to observe OI
7772 \AA\ in several places around the solar limb. We compared the observations
with synthetic spectra calculated with the RH code in the one-dimension
spherical geometry mode. New accurate hydrogen collisional rates were included
for the RH calculations. The observations reveal a dark gap in the lower
chromosphere, which is caused by variations in the line opacity as shown by our
models. The lower level of the 7772 \AA\ transition is populated by a downward
cascade from the continuum. We study the effect of Lyman-$\beta$ pumping and
hydrogen collisions between the triplet and quintet system in OI. Both have a
small but non-negligible influence on the line intensity.
