To remove fresh oil from old flagstones, use cat litter to absorb the liquid then use a detergent and scrubbing brush.
For old oil which has soaked in, you need to use a solvent such as paraffin to help. Leave the paraffin on for a few minutes then use the scrubbing brush.
Do not be tempted to use a wire brush or any other abrasive. It will leave a mark you'll never get rid of.