Observed ﹩{{\rm{H}}}_{\alpha }﹩ flux excess for emission spectra with respect to the PCA reconstruction. The horizontal axis shows the differences in mean ﹩{{\rm{H}}}_{\alpha }﹩ fluxes in λ6571–6557 Å between the observed LAMOST spectra and the reconstructed PCA spectra. The vertical axis shows the reduced χ² across the wavelength range that encapsulates the ﹩{{\rm{H}}}_{\alpha }﹩ features (λ6400–6550 Å, λ6590–6690 Å). Spectra with emission lines exhibit a large difference in the ﹩{{\rm{H}}}_{\alpha }﹩ line between the LAMOST and PCA-reconstructed spectra and are clearly separated from the majority of stars. The vertical dashed line delineates our criterion to select OB spectra with emission lines.