Distribution of the LAMOST DR5 stars with an S/N > 15 at the g band (the contours) in the Ca II K (3933 Å) vs. Hγ (4340 Å) plane. The blue dashed line with the asterisks and red dashed line with the unfilled circles represent the loci of the main-sequence and giant stars provided by Liu et al. (2015), respectively. The red solid lines are the cuts for OB candidate, following Equations (2) and (3).