We investigated decay properties of the broad resonance around 10 MeV in C-12 and measured inelastic scattering of 386 MeV alpha particles on O-16 at extremely forward angles including 0 degrees. We have found that the lower component of the broad 0(+) resonance at 10 MeV in C-12 decayed to the Be-8 + alpha channel, and that the higher component decayed both to the Be-8 + alpha channel and to the Be-8(2(+)) + alpha channel. On the O-16 nucleus, we have found two 0(+) resonances at 16.7 MeV and 18.8 MeV, which are near the 4 alpha breakup threshold. On the analogy to the broad resonances near the 3 alpha threshold in C-12, they are considered to have the C-12(0(2)(+)) + alpha cluster and the Be-8 + Be-8 cluster structures, respectively. It is consistent with the result of the experiment for the C-12(He-4,Be-8)Be-8 and C-12(He-4,C-12* -> Be-8 + alpha)alpha reactions.