Heating oil, or oil heat, is a low viscosity, liquid petroleum product used as a fuel for furnaces or boilers in buildings. Home heating oil is often abbreviated as HHO

Heating oil is commonly delivered by tank truck to residential, commercial and municipal buildings and stored in above-ground storage tanks ("ASTs") located in the basements, garages, or outside adjacent to the building. It is sometimes stored in underground storage tanks (or "USTs") but less often than ASTs. ASTs are used for smaller installations due to the lower cost factor. Heating oil is less commonly used as an industrial fuel or for power generation.

Red dyes are usually added, resulting in its "red diesel" name in countries like the United Kingdom. Solvent Yellow 124 is added as a "Euromarker" since 2002 in European Union.

Heating oil is very similar to diesel fuel, and both are classified as distillates. It consists of a mixture of petroleum-derived hydrocarbons in the 14- to 20-carbon atom range. During oil distillation, it condenses at between 250 and 350 °C (482 and 662 °F). Heating oil condenses at a lower temperature than the heavy (C20+) hydrocarbons such as petroleum jelly, bitumen, candle wax, and lubricating oil, which condense between 340–400 °C (644–752 °F). But it condenses at a higher temperature than kerosene, which condenses between 160–250 °C (320–482 °F).