This o-ring is for your SNOTTY ONE nasal aspirator.
The large green o-ring will seal between the cup (C) and case (D)
Please use silicone grease ONLY to lubricate and preserve the o-rings, this should be done at least once every 3-6 months depending on usage. Silicone grease will not swell or soften the rubber, which hydrocarbon based greases will do. NEVER use hydrocarbon based greases on the o-rings.