  We present a complete study of the associated production of the $\eta_c$
meson with light hadrons in $e^+e^-$ collisions at the B-factory energy, which
is demonstrated to be one of the best laboratories for testing the colour-octet
(CO) mechanism. The colour-siglet contributions are evaluated up to
$O(\alpha^2\alpha_s^3)$ while the CO ones are evaluated up to
$O(\alpha^2\alpha_s^2)$. For the first time, the angular distribution of the
$^1S_0^{[8]}$ production is studied at QCD next-to-leading order. We find that
the $^1S_0^{[8]}$ channel dominate the total cross section, while the
$^1P_1^{[8]}$ one exhibit its importance in the angular distribution, which
turns out to be downward going with respect to $\mathrm{cos}\theta$. This can
be considered as the most distinct signal for the CO mechanism.
