The decay of B mesons can be systematically predicted within the Heavy Quark Expansion. The leading term in the expansion corresponds to the decay of a free bottom quark, while higher order corrections are suppressed by powers of $1/m_b$. In my talk I report on the calculation of the NNLO corrections to nonleptonic decays of a free bottom quark considering the four-quark operators $O_1$ and $O_2$. In particular I will address several challenges like the treatment of $\gamma_5$ in dimensional regularization and the semi-analytic calculation of the master integrals.
