How can you remove candle wax from an oil painting?

If the oil painting has dried completely or is varnished (has a sheen to it), and the crayon mark isn't too large, wiping it with a soft warm damp cloth that has a drop or two of Murphy's oil soap on it could get it off. Crayon is wax and once an oil painting/varnish has dried, it is a pretty tough surface, so you might also try blotting the crayon with a soft cloth that has been soaked in really hot water, and then scraping the softened crayon off with your fingernail.