Barre:
Your question is not as clear as I would like it.
There are many stages between oil podced and dry clean oil fed into a topping unit.
Very simply, oil is produced in many different forms. It could range between high viscosity and condensate, high gas void fraction or some entrained gas, high water cut or dry, etc. These conditions dictate the process but usually, free water and gas are removed in a production separator resulting in wet foamy gas, oil based emulsion and water based emulsion. The oil base emulsion is dehydrated using any or all of the following processes in order of cost: residence time, heat, chemicals, mechanical separation.
This could mean a storage or slop tank, heater treater, dehydrator, etc. Once the water content is reduced to less than 1% it is put through an electrostatic dehydrator or desalter where chemicals are usually added and electrical grids are used to polarize and speed up the emulsion breaking. On which step of the process do you need more information?
Half of this is upstream and half is downstream.