In the past, during OOW Oracle published the official dates of OOW of the following year, but in the last few years they stopped doing this, so we can’t be sure when OOW19 will be. There was a lot of unknown about OOW18 until the official dates were released (and 2018 is the first year the User Group Sunday was removed and it became a 4-day event)
However, there are a few sites where you can see conference booking. A website called Convention Calendar states that OOW will be on September 15-19. However, Moscone Center website says September 16-18.
Based on that I can assume that OOW19 will be quite early next year, but we cannot know if it will be back to a 5-day event, will be shortened to a 3-day event, or will stay a 4-day as this year. I guess we will have to wait a bit for more information.