Probably there is already a similar thread to this one but here goes nothing
If an oasis is inside your village 3x3 you are entitled to the full 25% / 25%+25% / 50% bonus.
You also get to station there 250 units boosting your bonus.
No more ranks, no more silly wars, everyone is happy with rank 1 and the main reason I'm proposing this is because wars tend to be between the same kingdom members and not against other kingdoms.
You still need to fight for the oasis itself and make sure it belongs to your kingdom so you can benefit from the troop bonus but it doesn't make sense to have internal brawls because of the rank and of how far you are to the oasis because if it is within your 3x3 you can use it.
This will also lead to you finding more troops stationed at the oasis as for the defensive bonus of the oasis I don't know how they are attributed but they seem random so they could still be randomly given.