  We propose a holographic dual of large N_c QCD with the gauge group O(N_c)
and USp(N_c) and N_f flavors of massless quarks. This is constructed by adding
O6-planes to an intersecting D4-D8 system in type IIA superstring theory. The
holographic dual description is formulated in the Witten's D4-brane background
with D8-branes and O6-planes embedded in it as probes. The D4-brane background
gives rise to a smooth interpolation of D8-anti-D8 pairs and an O6-anti-O6
pair. We show that the resultant brane configuration explains geometrically the
flavor symmetry breaking patterns in O(N_c) and USp(N_c) QCD that are caused by
quark bilinear condensates.
  We next discuss that baryons can be realized as D4-anti-D4 pairs wrapped on
S^4 that intersect with the O6-plane. By analyzing the tachyons on it, we
reproduce the stability conditions of the baryons that are expected from the
gauge theory viewpoint. The stable baryon configurations are classified
systematically by using K-theory. We also give a similar analysis for the flux
tubes and again reproduce the results that are consistent with QCD.
