Observed (red line) and theoretical (black line) centre-to-limb variation of the O I 777 nm triplet using the 3D hydrodynamic model solar atmosphere and our best description for the inelastic O+H collisions (Sect. 2.4.2). The oxygen abundance was fit for μ = 1.0, and fixed to this value for the other μ-pointings. In the top panel the observational data used in the fit (blue dots) have been subsampled by a factor of two to make it easier to see the theoretical spectra below it. In the bottom panel, the residual is defined as the theoretical minus the observed spectra.