  We have observed an excited state in the neutron-rich semi-magic nucleus O-23
for the first time. No such states have been found in previous searches using
gamma-ray spectroscopy. The observation of a resonance in n-fragment
coincidence measurements confirms the speculation in the literature that the
lowest excited state is neutron unbound and establishes positive evidence for a
2.8(1) MeV excitation energy of the first excited state in O-23. The
non-observation of a predicted second excited state is explained assuming
selectivity of inner-shell knockout reactions.
