The charge-state distribution and the energy loss of oxygen ions in a laser-produced hydrogen plasma have been investigated experimentally. The plasma target had a maximum electron density of 5 × 1018 cm−3 and a maximum electron temperature of 13 eV. The mean charge state of the oxygen ions in the hydrogen plasma was measured to be 5.1, which was considerably higher than that in hydrogen gas. The energy loss of oxygen ions in the plasma of a density less than 1 × 1018 cm−3 in the plasma showed a large enhancement compared with that in hydrogen gas. However, the energy loss in the plasma of a density above 1 × 1018 cm−3 showed no enhancement.