o-Carboryne (1,2-dehydro-o-carborane) is a very useful synthon for the synthesis of a variety of carborane-functionalized heterocycles. Reaction of o-carboryne with N-protected indoles gave carborane-fused indolines if the protecting group was TMS via dearomative [2 + 2] cycloaddition or carboranyl indoles for N-alkyl ones through formal C-H insertion reaction. For N-aryl indoles, both reactions were observed, giving two products, in which the product ratio was dependent upon the nature of the substituents on the aryl rings. In general, electron-withdrawing substituents favor [2 + 2] cycloaddition, whereas electron-donating substituents promote a formal C-H insertion pathway. This reaction is also compatible with other heteroaromatics. Thus, a stepwise reaction mechanism was proposed to account for the experimental observations. These protocols offer general and efficient methods for the preparation of carborane-functionalized indoles and indolines as well as other heterocycles. The observed dearomative [2 + 2] cycloaddition represents the first example of indoles to undergo such reaction in the absence of transition metals or without UV irradiation. All new compounds were fully characterized by H-1, C-13, and B-11 NMR spectroscopy as well as HRMS spectrometry. Some were further confirmed by single-crystal X-ray analyses.