Part of OK-25 package, but just the one size.  Sold in different quantities with or without silicone grease.  Use silicone grease to delay o-ring rot.  Replace every 3rd filter change or as needed.

Caution:  Fits the new OB1 housing with an original Black o-ring. Older OB1 with original Blue o-ring needs smaller o-ring, part of K-4 package.

Caution:  Fits the older U25 Series housing.  The newer housings use a thicker 3/16" inch o-ring.