  We calculate bosonic massive closed string states at arbitrary mass levels
scattered from Orientifold planes in the high-energy, fixed angle limit. For
the case of O-particle scatterings, we obtain infinite linear relations among
high-energy scattering amplitudes of different string states. We also confirm
that there exist only closed string Regge poles in the form factor of the
O-particle amplitudes as expected. For the case of O-domain-wall scatterings,
we find that, like the well-known D-instanton scatterings, the amplitudes
behave like field theory scatterings, namely UV power-law without infinite
Regge poles. In addition, we discover that there exist only finite number of
t-channel closed string poles in the form factor of O-domain-wall scatterings,
and the masses of the poles are bounded by the masses of the external legs. We
thus confirm that all massive closed string states do couple to the
O-domain-wall.
