The structure of a neutron-rich 25F nucleus is investigated by a quasifree (p,2p) knockout reaction at 270 A MeV in inverse kinematics. The sum of spectroscopic factors of π0d5/2 orbital is found to be 1.0 ± 0.3 . However, the spectroscopic factor with residual 24O nucleus being in the ground state is found to be only 0.36 ± 0.13 , while those in the excited state is 0.65 ± 0.25 . The result shows that the 24O core of 25F nucleus significantly differs from a free 24O nucleus, and the core consists of ∼ 35 % 24Og.s. and ∼ 65 % excited 24O. The result may infer that the addition of the 0d5/2 proton considerably changes neutron structure in 25F from that in 24O, which could be a possible mechanism responsible for the oxygen dripline anomaly.