  We study the possible impact of nuclear effects and final state interactions
on the determination of the oscillation parameters due to mis-reconstruction of
non-quasi-elastic events as quasi-elastic events at low energies. We analyze a
$\nu_\mu$ disappearance experiment using a water \v{C}erenkov detector. We find
that, if completely ignored in the fit, nuclear effects can induce a
significant bias in the determination of atmospheric oscillation parameters,
particularly for the atmospheric mixing angle. Even after inclusion of a near
detector a bias in the determination of the atmospheric mixing angle comparable
to the statistical error remains.
