First Oasis = Level 10 Hero's Mansion
Second Oasis = Level 15 Hero's Mansion
Third Oasis = Level 20 Hero's Mansion
You can use the same hero to get three oases. You DO NOT need to create another hero.
You can only ever have ONE active hero; however, it's a good idea to create him from one of your better troops types.
Once a hero dies, you can resurrect him. You can have three inactive hero's of different troop types (or two inactive and one active), but only one active hero.
You can only conquer oases within your 7x7 area.
Hope this helps!
Sa
P.S. I had an oasis and it was just conquered by someone from LA....