I've found that oasis rank is often a problem or annoyance for capital cities, particularly within your own kingdom. I'm sure many have have been in situations were a kingdom member has forced the nr 1 rank of your cropper capital oasis by settling villages/cities next to the oasis and/or parking large amount of troops in the oasis. Often for a regular village or a 6c capital with low level fields that would recieve a much lower production boost from that oasis than you would.
I would suggestion the following
Instead of oasis influence being affected by village population and the number of troops inside:
1. Oasis influence is based on the production bonus the village would recieve (i.e. total base production). That is, the village that would recieve the higher bonus from the oasis automatically gets rank 1, the second highest, rank 2 and so on.
2. Remove gaining influence and resources from stationing troops inside oasis
With this system you have 2 ways to increase in rank:
1. Increase the level of your fields untill it is higher than your opponent (you would do this with allies)
2. Catapult the other players fields so your fields are higher (this is solution for enemies)
This way, the player that would recieve the highest bonus from the oasis automatically gets it (great for kingdom) and it promotes players to upgrade their fields which is good in the long term. Additionally, we will no longer need to keep worthless troops sitting around in the oasis.
In cases were 2 players are in a tie, for instance two 15c capitals with lvl 18 fields, the one who assigned the oasis first gets the higher rank. That way the guy that was there first gets the highest rank but he still has the option to give it to the other guy if he wants by removing oasis and reassigning it again.