They are afraid of the dark. Seriously, have you ever tried to find one at night? A lot of people attribute this to lack of vision, but it's been theorized that they tend to congregate near one another for comfort during the nighttime hours.
If you happen to find one during the day, and it has not run away from you (Because you followed the instructions in the previous oasis status) you should probably light a fire before going to sleep. This will insure your new found oasis will remain at your side for the duration of the night. This may also result in what we like to call "Islanding", where you might wake up on a small island, surrounded on all sides by the oasis. If this happens, do not be alarmed. It is simply the oasis' way of hugging you for providing it with a light source during the frightening night.