  We study a "heavy" QCD axion whose coupling to the standard model is
dominated by $a G \widetilde{G}$ but with $m_a \gg m_\pi f_\pi / f_a$. This is
well motivated as it can solve the strong CP problem while evading the axion
quality problem. It also poses interesting challenges for its experimental
search due to its suppressed couplings to photons and leptons. Such axion with
mass around a GeV is kinematically inaccessible or poorly constrained by most
experimental probes except B-factories. We study $B \to K a$ transitions as a
powerful probe of the heavy QCD axion by performing necessary 2-loop
calculations for the first time, together with some improvement on the existing
analysis strategy. We find some of the existing limits are enhanced by at least
an order of magnitude. We also demonstrate that the bounds are robust against
unknown UV physics. For forthcoming data sets of the Belle II experiment, we
provide a projection that $f_a$ of a few TeV is within its future reach, which
is relevant to the quality problem.
