When we developed the first village we search near this village a location with 15 crops. If we find more we will try to choose the best of them: we will look on a radius of 3 square who have more oases and who give the best bonus.

 

Attention: to take oases of clay, wood or iron on a capital with 15 crop is a big waste because you could take from a town with 4-5 mines with these type of resources and you will gain a lot.

Why to take oases of crop on a village with 15 crop fields?
  On a capital with 15 crops all at level 15, without any oases of crop we will gain a production of 18.000 crop / hour. On an 6 crop village we will obtain just 7200 crop / hour.

Without oases:
   Crop level  Village 15 crops    Village 6 crops
      15             18.000                7.200
      16             22.500                9.000
      17             29.250              11.700
      18             36.000              14.400  
     
If we have a capital with 15 crops, all at level 18 with 3 oases with a bonus of 50% each, we have a production of 72.000 crop / hour.       
     
Production of crop represent the base production + bonus. Bakery and grain mill will give you each up to 25% bonus. Bonus gained from oases is added to the base production not to all production of crop.