Why did I not get the oasis?

i attacked, it said i won but i didn't get it

Perhaps you are trying to capture more than your level allows you to have. You can only have a certain amount of oases per level.

Your Level - Oases

Level 1 - 1
Level 2 - 1
Level 3 - 2
Level 4 - 2
Level 5 - 3
Level 6 - 3
Level 7 - 4
Level 8 - 4
Level 9 - 5
Level 10 - 5
Level 11 - 6
Level 12 - 6
Level 13 - 7
Level 14 - 7
Level 15 - 8
Level 16 - 8
Level 17 - 9
Level 18 - 9
Level 19 - 10
Level 20 - 10

Another way to know how many oases you can have per level is to divide your current level by two and round up. E.g at level 5 you can have 3, because half of 5 is 2.5, round up and it's 3.