[en] The structure of the ¹⁸O nucleus at excitation energies above the α decay threshold was studied using ¹⁴C+α resonance elastic scattering and (⁷Li,t) a-transfer reactions. A number of states with large α reduced widths have been observed, indicating that the a-cluster degree of freedom plays an important role in this N≠Z nucleus. A 0⁺ state with an α reduced width exceeding the single particle limit was identified at an excitation energy of 9.9±0.3 MeV. We discuss evidence that states of this kind are common in light nuclei and give possible explanations of this feature. Also, the astrophysical implications of the cluster structure of ¹⁸O is discussed.