  We write the vertex operators of massless NS-NS and RR states of Type II
superstring theory in the presence of Orientifold p-planes. They include the
usual vertex operators of Type II theory and their images. We then calculate
the two-point functions of these vertex operators at the projective plane PR_2
level. We show that the result can be written in the Veneziano-type formulae,
with the same kinematic factor that appears in the D_p-branes amplitudes. While
the scattering amplitudes with the usual vertex operators are not gauge
invariant, the above amplitudes are invariant. From the amplitude describing
scattering of two NS-NS states off the O-plane, we find the low energy
effective action of O-planes. The result shows a relative factor 2^{p-6}
between couplings to O-planes and to D-branes at (\alpha')^2 order.
