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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 OI7772 \AA\ in several places around the solar limb. We compared the observationswith synthetic spectra calculated with the RH code in the one-dimensionspherical geometry mode. New accurate hydrogen collisional rates were includedfor the RH calculations. The observations reveal a dark gap in the lowerchromosphere, which is caused by variations in the line opacity as shown by ourmodels. The lower level of the 7772 \AA\ transition is populated by a downwardcascade from the continuum. We study the effect of Lyman-$\beta$ pumping andhydrogen collisions between the triplet and quintet system in OI. Both have asmall but non-negligible influence on the line intensity.

Solar off-limb emission of the OI 7772 \AA\ line