  We present 22\,901 OB spectra of 16\,032 stars identified from LAMOST DR5
dataset. A larger sample of OB candidates are firstly selected from the
distributions in the spectral line indices space. Then all 22\,901 OB spectra
are identified by manual inspection. Based on a sub-sample validation, we find
that the completeness of the OB spectra reaches about $89\pm22$\% for the stars
with spectral type earlier than B7, while around $57\pm16$\% B8--B9 stars are
identified. The smaller completeness for late B stars is lead to the difficulty
to discriminate them from A0--A1 type stars. The sub-classes of the OB samples
are determined using the software package MKCLASS. With a careful validation
using 646 sub-samples, we find that MKCLASS can give fairly reliable sub-types
and luminosity class for most of the OB stars. The uncertainty of the spectral
sub-type is around 1 sub-type and the uncertainty of the luminosity class is
around 1 level. However, about 40\% of the OB stars are failed to be assigned
to any class by MKCLASS and a few spectra are significantly misclassified by
MKCLASS. This is likely because that the template spectra of MKCLASS are
selected from nearby stars in the solar neighborhood, while the OB stars in
this work are mostly located in the outer disk and may have lower metallicity.
The rotation of the OB stars may also be responsible for the
mis-classifications. Moreover, we find that the spectral and luminosity classes
of the OB stars located in the Galactic latitude larger than 20$^\circ$ are
substantially different with those located in latitude smaller than 20$^\circ$,
which may either due to the observational selection effect or hint a different
origin of the high Galactic latitude OB stars.
