Phosphorus‐containing heterocycles have evolved from laboratory curiosities to functional components, such as ligands in catalytically active metal complexes or molecular constituents in electronic devices. The straightforward synthesis of functionalized heterocycles on a larger scale remains a challenge. Herein, we report the use of the phosphaethynolate (OCP)− anion as a building block for various sterically unprotected and functionalized hydroxy substituted phosphorus heterocycles. Because the resulting heterocycles are themselves anions, they are building blocks in their own right and allow further facile functionalization. This property may be of interest in coordination chemistry and material science. Small, but effective—such is the nature of the OCP− anion of the sodium phosphaethynolate salt. In atom‐economic cycloadditions, several five‐ and six‐membered phosphorus heterocycles are accessed, all of which are useful building blocks in their own right.