The Wise Oak + Dying Sun

My res before using flasks :
Fire : 75
Cold : 85
Lightning : 65
So if I use The Wise Oak I'll get Cold Pen, but what happens if I use Dying Sun after The Wise Oak, will it change to Fire Pen or will it stay as Cold Pen?

I believe Wise Oak's effect is "live". If your resistances change during its effect, it will change penetration and damage reduction accordingly.
So, I expect you will get ColdPene until you pop a Dying Sun, after which you'll get a FirePene