Left and right panels display the Ca II K vs. Hγ and Fe vs. Hγ distribution of the OB stars found in an arbitrarily selected 5000 spectra with an S/N > 15 and within Galactic latitude of 20°. The filled circles are OB stars identified with the previous approach, while the filled triangles are those failed to be identified. The colors code the spectral subtypes of these spectra according to the criteria of Table 2. The red lines in both panels stand for the same selection criteria used in Figures 1 and 2, respectively.