The unbound excited states of the neutron drip-line isotope 24 O have been investigated via the 24 O(p,p ) 23 O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24 O * was reconstructed from the momenta of 23 O and the neutron. The spinparity of the first excited state, observed at Ex = 4.65 ± 0.14 MeV, was determined to be J π = 2 + from the angular distribution of the cross section. Higher-lying states were also observed. The quadrupole transition parameter β2 of the 2 Otsuka et al. have investigated theoretically the structural evolution of the oxygen isotopes with increasing neutron number (N ) and attributed the development of the shell closure at N = 16 to the strong neutron-proton tensor interaction [10,11].In this Letter we report on the first spectroscopic study of 24 O by proton inelastic scattering. In addition to the excitation energies of the states populated, the wellknown character of proton inelastic scattering also permits the spins-parities, as well as the quadrupole transition parameter (β 2 ) of the first 2 + state to be deduced. As described below, we have been able to provide a firm 2 + assignment for the state at E x = 4.65 ± 0.14 MeV and determine the β 2 , the small value of which is indicative of the spherical closed-shell character of 24 O. A comparison of the E x (2 + 1 ) and β 2 for the chain of oxygen isotopes shows strong evidence for a large shell gap at N = 16.The experiment was performed at the RIPS facility [17] at RIKEN. A schematic view of the downstream section of RIPS and the experimental setup is shown in Fig. 1

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