A winter (DJFM) North Atlantic Oscillation index (NAOi) has been reconstructed over the peridod 1781-1996 by calibrating the winter frequencies of Lamb weather types to the winter PC-based NAOi of Hurrell (1995, doi:10.1126/science.269.5224.676).The data file contains 4 columns: year of JFM, NAOi, uncertainty (1 std), number of unclassified days over this winter.The NAOi has no unit since regressed to a normalised index. Note that this index has not been renormalised (to an average value of zero and standard deviation of 1).The number of unclassified days directly affects the reliability and consistency of the NAOi. For instance, Delaygue et al. (2018) require less than 24 unclassified days per winter to consider the NAOi.