O-rings in filter housings sit in grooves at the bowl rim and the head. They fail when they dry out, when grit gets caught in the groove, when the bowl is twisted during removal, or when overtightening flattens the ring. Each filter change is the moment to look at them, and the seal check is easy to skip because nothing is visibly wrong until the cabinet floor is wet.
What to inspect at every filter change
Remove the bowl and wipe the O-ring and its groove with a clean, lint-free cloth. Look for flattening, cracks, cuts, twists or grit. Compare the old ring with the replacement ring. If the old ring looks damaged, replace it rather than trying to save it, because a ring that has taken a set will not reseal reliably.
Use only the lubricant the housing maker names. Apply a thin film, not a glob, since excess lubricant can carry grit back into the groove. Seat the bowl by hand until it stops, then tighten it only as far as the maker's instructions describe. Avoid using a wrench unless the maker calls for one, because overtightening is a common cause of cracked bowls and flattened rings.
Who owns the check
Name one person. That may be you if you change the filters yourself, the installer if they handle changes, or the service provider under a contract. Write down who checks the O-rings, who replaces spares, and who records the work. If you use a provider, ask before you sign whether a seal check is part of each visit or only a filter swap.
What to keep on hand
Keep a correct O-ring set for every housing, the lubricant the maker specifies, and a bowl wrench if the housing uses one. Label each housing with its ring size or part number so a replacement goes back to the right place. A spare set in the cabinet saves a rushed trip to a store when a ring fails on a weekend.
When the ring is not the problem
If a bowl still weeps after a fresh ring is seated, check the threads on the bowl and head for damage, look for hairline cracks in the bowl, and check whether the housing sees pressure swings from a failing pump or pressure tank. Those are different jobs, and tightening harder will not fix any of them. Stop, note what you found, and bring it to whoever owns the system.