From Oasis:

For complete confidence that your guitar is getting the humidification it needs in dry seasons without worrying about your humidfier leaking or hurting your instrument, check out the original blue Oasis® guitar humidifier. The OH-1 guitar humidifier is a soundhole humidifier designed to maintain 45-55% relative humidity inside the guitar body when the humidity outside the case is between 25-40%, provided that the guitar is already hydrated. The OH-1 is not designed to restore humidity to a dried out guitar. The OH-1 is our most popular model. Each Oasis® OH-1 comes with a stabilizer bar for mounting the humidifier and a 10 cc. syringe for distilled water filling control.

Because tap and bottled water have dissolved minerals that can clog the liner, distilled water is recommended to extend the useful life of your humidifier.