Spoiler:
Take water for 4 days, leave the oasis.
After 1 day, leave water for 2 days in a safe place, use the remaining water to get back to the oasis. (2 days have passed)
Take water for 4 days, leave the oasis again.
After 1 day, you are left with 3 days' water, but find the 2 day's water you left (I hope no one stole it), so you have water for 5 days. (3 days have passed)
Go on for one more day (distance of two days). You have now water for 4 days. Hide water for 2 days there, use the rest of it to go back to the oasis. (6 days have passed)
Take water for 4 days, leave the oasis for the last time.
After 2 days, you have 2 days' water left, but find the water you hid, so you now have water for 4 more days. With it, you can cross the desert. (12 days have passed)
12 days in total.