In Japanese, this word referrs to a snack to be eaten at 3:00 PM, whether it be watermelon, Jelly, cookies, etc.
Oyatsu also means "the eighth."
The origin of this word comes from the Chinese Zodiac, and as the chart at that node shows, 3:00 PM is in the middle of the eigth "hour", the time of the Ram. The bells at temples would be rung once every 120 minutes to let people know the time. For some reason, the custom of eating at the 8th "hour" stayed.