Oil paint is best mixed with oil. There are different kinds on the market, however it is best to use something like linseed oil. That is what most oil paint is mixed with that you buy today. You can also use some type of paint thinner or turpentine. In deciding which to use, you need to know the different results they produce.
If you use oil, the paint will take a little longer to dry. In adding the oil, you are diluting the paint pigments somewhat and will thus lengthen the drying process time. It also will add more of a sheen to the finished work, because oil shines.
If you use a paint thinner, mineral spirits, turpentine, or some other solvent, it will speed up the drying process. It will also result in more of a matte or dull finish; the opposite of what oil produces. If you use too much of a solvent to thin your painting, it will begin to crack over time. You don't want that.
Artists can use these differences to their advantage. It all depends on what you are trying to achieve in the form of art expression. I myself, prefer to use linseed oil. It doesn't change the composition of the oil paint since that is already an ingredient. It only changes the ratio.
Also, if you use oil your finished work will retain the quality and preservation we all want.