Prisons
Captured troops would be sent to prisons. Prisoners would then be made to maintain oasis that you own, reducing the rate at which they drop level.
If you get enough prisoners then the oasis would stop dropping level and even start gaining level (up to level 10).
However, prisoners would consume crop.
You'd be able to upgrade a prison to level 20 and the number of prisoners held would determine how much maintenance is done on oasis.
You could disband prisoners (set them free, they return to their owner) to reduce your crop consumption (but also reduce amount of maintenance on oasis).
This also means that the player who owned the prisoners would suddenly get increased crop consumption unexpectedly

Alternatively, released prisoners could join barbarian camps (fortified points) or unoccupied oasis making them better defended and harder to capture.
If a city starves, the prisoners starve first, before the player's army.