We have assembled a complete list of the most accurately measured wavelengths for all classified lines of O II. The data are based mainly on recent extensions of the observations and analysis of this spectrum carried out at the University of Lund, Sweden. We derived new optimal values for the energy levels using a computer code and the observed wavelengths for all classified lines. Relevant astrophysical wavelength measurements, appropriately weighted were included in the level‐optimization calculation. The tabulated data include about 1000 observed lines (376–11 663 A˚) classified as transitions between 125 odd‐parity and 133 even‐parity levels. In addition to the observed wavelength values, wavelengths calculated from wavenumber differences of the levels are given for all observed lines and for more than 200 predicted lines that have not yet been observed. The calculated wavelengths are generally more accurate than the observed values, the most accurate calculated values (uncertainties 0.0005 to 0.0020 A˚) being in some cases more accurate than the observed wavelengths by up to an order of magnitude. Vacuum wavelengths are given for all lines, and wavelengths in air are also included for the region above 2000 A˚. §