What organic solvent did you try? At the end it will depend on how much cobalt sulfate you want to be dissolved in the oil. You could try ethanol or dmso. 

I am trying to have a 3% cobalt sulfate in walnut oil or linseed oil though. I wonder that ethanol or DMSO is able to dissolve into that oil. Thus I will use SPAN #80 as a mediator first to mix  with cobalt sulfate then into oil.

You need to consider that surfactant agents will not really contribute to dissolve (to obtain a molecular or ionic dispersion) your salt but only it will help to obtain a coarse dispersion it in the pure oil. Nevertheless, it could work if the viscosity is increased.