Slab model prediction for the [O I] 557 nm/[O I] 630 nm ratio for collisions with electrons. Solid lines refer to gas temperatures of 5, 8 and 10 × 10³ K. The cyan area indicates the location of the range of values observed in the NLVC for the sub-sample of sources where the [O I] 557 nm line has been detected in this component. The grey area indicates instead the location of the detected ratios in the HVC.