The impacts caused by the polarity reversal of the applied voltage on space charge dynamics and electric field enhancement across an oil gap have been studied in an oil gap and single-layer pressboard composite insulation system. The experimental results validate that the electric field enhancement across the oil gap immediately after the application of the reversed electric field is directly dependent on the number of residual charges within the insulation system at the moment of the application of the reversed stress. Therefore, the electric field enhancement in the system is strongly dependent on the magnitude of the applied electric field, the duration of polarity reversal and the degradation of the mineral oil. An estimation of the enhanced electric field immediately after polarity reversal based on the charge decay characteristics has been proposed and successfully applied to calculate the electric field enhancement in an aged oil sample under 20 kV/mm, which cannot be accurately measured due to the very fast space charge dynamics in the aged oil under a strongly enhanced electric field.