  We have determined the Fermi surface in KOs$_2$O$_6$ ($T_c$ = 9.6 K and
$B_{c2} \sim$ 32 T) via de Haas-van Alphen (dHvA) oscillation measurements and
a band structure calculation. We find effective masses up to 26(1) $m_e$ ($m_e$
is the free electron mass), which are unusually heavy for compounds where the
mass enhancement is mostly due to electron-phonon interactions. Orbit-resolved
mass enhancement parameters $\lambda_{dHvA}$ are large but fairly homogeneous,
concentrated in the range 5 -- 8. We discuss origins of the large homogeneous
mass enhancement in terms of rattling motion of the K ions.
