The O ring comes off with a flat head screw driver. After removing the O ring, insert the new O ring on the cap (my oil filter came with with the O ring). Make sure to rub some old oil on the rubber ring first. Insert the filter and hand screw the cap back into the casing. The oil filter cap says 25Nm for use with a torque Wrench. I did not use a torque wrench. It was fairly easy to detect when the cap was fully seated. If you choose to forego the torque wrench - BE CAREFUL not to over tighten!