The idea behind making oases available for up to 5 players is to encourage cooperation.
There are 4 basic scenarios. I'm talking about strategies, not implementation. For capital oases it's a huge deal.
First, the easiest, is when an oasis is located within the player's kingdom. By placing certain number of troops players regulate somehow who gets what bonus.
Second, also pretty obvious. An oasis is located in unallied/unfriendly kingdom. You fight for it.
If oasis is located in free territory, not much can be done. Whoever is closer will get the higher bonus.
The most complicated situation is when an oasis is located in friendly kingdom. You can't attack it and amount of troops you station there does not make any difference.
What is your experience resolving those situations? Especially the last one. Did it increased the level of cooperation (real experience only please) or just created trouble?
Eventually, long term alliances will develop in kingdoms environment (let's be optimistic) and they will create policies regarding oases bonus distribution. What rules you find appropriate?