Damaged cylinder barrel.
Replace cylinder barrel. Correct cause of barrel damage.
Rod seal leaking.
Replace seal. If contamination caused seal to wear, look for source. Wear may be caused by external as well as internal contaminants. Check piston rod for scratches or misalignment.
Tighten parts until leakage has stopped.
Piston rod damaged.
Check rod for nicks or scratches that could cause seal damage or allow oil leakage. Replace defective rods.