Abstract : The unbound excited states of the most neutron-rich dripline oxygen isotope, O (Z=8, N=16), have been investigated using the 24O(p, p′)O* reaction at the beam energy of 63 MeV/nucleon in inverse kinematics at RIKEN. The 24O beam was produced in the projectile-like fragmentation process of a 95 MeV/nucleon 40Ar primary beam on the Be production target. The decay energy spectrum of 24O* in 23O+n channel was reconstructed in the invariant mass of the neutron and the charged fragment detected in coincidence. The first unbound excited state of 24O has been observed at Ex=4.68 -0.14 +0.21 14 MeV (preliminary) along with the evidence for another higher lying state at around 7.3 MeV. The energy of the former state is compared with existing data and the results of shell-model calculations.