Departure coefficients of the O i IR triplet levels as a function of the optical depth for three cases of the fine-structure treatment (see text for details). The black solid line shows the results obtained when 3p⁵P is treated as a superlevel (case 1); colored lines show the results when fine-structure splitting is included; dashed lines show case 2 (no collisional coupling between fine-structure levels); solid lines show case 3 (strong collisional coupling between fine-structure levels). Top and bottom panels refer to terms 3p⁵P (upper level of the triplet) and 3s⁵S^o (lower level of the triplet), respectively.