Direct answer: Tar oil is a general name for oily fractions obtained from tar or tar distillation. Its meaning depends strongly on whether the source is coal tar, wood tar, or another tar.
When crude tar is distilled, different fractions come off at different temperatures. Some are lighter oils, some are heavier oils, and some residues become pitch. Coal-tar oils and wood-tar oils differ in chemistry, smell, hazard profile, and use.
Tar oils appear in creosote, wood preservatives, industrial chemistry, perfumery ingredients, old technical recipes, and chemical catalogues. The phrase should always be tied to a source and context before discussing use.
Tar oil is not the same as raw tar. It is usually a separated fraction or oil-rich portion. It is also not a safe generic household oil. Some tar oils are regulated, hazardous, or unsuitable for direct contact.
In short: Usually an oily fraction from tar. Can come from wood tar, coal tar, or other tar sources. May be related to creosote or specialist ingredients. The source determines meaning and risk.