Oil Red O is a lysochrome

Lysochrome

A lysochrome is a soluble dye used for biochemical staining of triglycerides, fatty acids, and lipoproteins. Lysochromes such as Sudan IV bind to the lipid in a substrate and show up as colored regions. The dye does not stick to any other substrates, so a quantification or qualification of lipid...

 (fat-soluble dye) diazo dye used for staining

Staining (biology)

Staining is an auxiliary technique used in microscopy to enhance contrast in the microscopic image. Stains and dyes are frequently used in biology and medicine to highlight structures in biological tissues for viewing, often with the aid of different microscopes...

 of neutral triglycerides and lipid

Lipid

Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...

s on frozen sections and some lipoproteins on paraffin sections. It has the appearance of a red powder with maximum absorption at 518(359) nm.
Oil Red O is one of the dyes used for Sudan stain

Sudan stain

Sudan staining is the use of Sudan dyes to stain sudanophilic substances, usually lipids. Sudan lysochromes are used....

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Similar dyes include Sudan III

Sudan III

Sudan III is a lysochrome diazo dye used for staining of triglycerides in frozen sections, and some protein bound lipids and lipoproteins on paraffin sections. It has the appearance of reddish brown crystals and a maximum absorption at 507 nm.Sudan III is a dye used for Sudan staining...

, Sudan IV

Sudan IV

Sudan IV is a lysochrome diazo dye used for the staining of lipids, triglycerides and lipoproteins on frozen paraffin sections. It has the appearance of reddish brown crystals with melting point 199 °C and maximum absorption at 520 nm.Sudan IV is one of the dyes used for Sudan staining. Similar...

, and Sudan Black B

Sudan Black B

Sudan Black B is a lysochrome diazo dye used for staining of neutral triglycerides and lipids on frozen sections and some lipoproteins on paraffin sections. It has the appearance of a dark brown to black powder with maximum absorption at 596-605 nm and melting point 120-124 °C...

. The staining has to be performed on fresh samples, as alcohol fixation removes the lipids.

Oil Red O largely replaced Sudan III and Sudan IV, as it provides much deeper red color and the stains are therefore much easier to see.

In pyrotechnics

Pyrotechnics

Pyrotechnics is the science of using materials capable of undergoing self-contained and self-sustained exothermic chemical reactions for the production of heat, light, gas, smoke and/or sound...

, Oil Red O is used in some compositions of red colored smoke

Colored smoke

Colored smoke is a kind of smoke created by an aerosol of small particles of a suitable pigment or dye.Colored smoke can be used for smoke signals, often in a military context. It can be produced by smoke grenades, or by various other pyrotechnical devices...

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