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