Why is it that when you pour oil on to hot water and then try to dissolve salt into it the salt dores not dissolve in it¿

Salt, NaCl is an ionic compound. Ionic compounds are not very soluble at all in non-polar solvents like oils. If you shake the mixture vigorously, the NaCl will migrate to the aqueous layer.

Normally, the oil is less dense than water and so rests on the top of the aqueous layer. When you add the salt to the mixture, the salt simply sits on top of the oil because, as Mississippichem has stated, the ionic salt will not dissolve in the non-polar oil.