This is a frequently asked question. The "O" Ring and Retainer Ring are both inside the main casting. If you run your fingernail along the inside edge of the main casting you will feel a groove. Pull up on the groove and you will remove the retainer ring. Under it is the "O" Ring.
The "O" Ring and Retainer Ring as well as the interior of the XL-2 Main Casting should be cleansed of debris each time the Diatom XL Filter is recharged, if you have taken the filter apart to clean it.
After removing the XL-4 Filter bag and the XL-5 Turbulence Tube from the casting, slide off the XL-9-1 Retainer Ring as described above. Remove the "O" Ring. Wash each of the parts and set them aside. Wash the Main Casting with a small brush and water spray to remove any Diatom Powder or other debris from inside the cavity.
Allow the parts to dry. Apply a thin coat of Vaseline or silicone grease to the "O" Ring and roll it back into place. Install the retainer ring and assemble the filter per assembly instructions. Following these instructions will assure a good seal.