The Olah reagent is a nucleophilic fluorinating agent. It consists of a mixture of 70% hydrogen fluoride

Hydrogen fluoride

Hydrogen fluoride is a chemical compound with the formula HF. This colorless gas is the principal industrial source of fluorine, often in the aqueous form as hydrofluoric acid, and thus is the precursor to many important compounds including pharmaceuticals and polymers . HF is widely used in the...

 and 30% pyridine

Pyridine

Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...

; alcohols react with this reagent to give alkyl fluorides:
It acts as a stabilized, less volatile form of hydrogen fluoride. It's used in the fluorination of steroids and in deprotection of peptides. Instead of hydrogen fluoride, several other flourinating agents can be used, such as diethylaminosulfur trifluoride (DAST).