The structure of a neutron-rich F25 nucleus is investigated by a quasifree (p,2p) knockout reaction at 270A 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 O24 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 O24 core of F25 nucleus significantly differs from a free O24 nucleus, and the core consists of ∼35% O24g.s.. and ∼65% excited O24. The result may infer that the addition of the 0d5/2 proton considerably changes neutron structure in F25 from that in O24, which could be a possible mechanism responsible for the oxygen dripline anomaly.