An oil bath is a laboratory heating device which uses boiling oil as the temperature regulator. Since different oils have different boiling points it is possible to obtain a temperature near to the desired temperature by selecting an oil with a boiling point as close as possible to the desired temperature.
An oil bath is essentially a container of oil heated by a hot plate, open flame or other heat source.
Another use of an oil bath is to filter particulates out of air, by leading the air stream through an (unheated) oil bath. This type of air filter was used in car engines, but has been replaced by modern paper air filters.