Is turpentine the only thing that can be mixed with linseed oil to make a medium when using oil paint in art?

No, Turpentine (Terabenthine) is traditional. I like the smell. Some people object to it or claim to be allergic. The alternative is mineral spirits, usually called odorless paint thinner. There is a new paint thinner which is milky white which I doubt is suitable. I think it dries clear.

Use odorless solvent instead - a basic medium formula is one third odorless solvent, one third linseed and one third damar varnish.