  Shell-model calculations in a large model space are performed for
_{\Lambda}^{16}O and_{\Lambda}^{17}O. Effective interactions with degrees of
freedom of \Sigma in addition to \Lambda and nucleons are derived from
hyperon-nucleon and nucleon-nucleon interactions within the framework of the
unitary-model-operator approach. Effects of the \Sigma degrees of freedom and
the parity-mixing intershell coupling on the \Lambda hypernuclear structure are
investigated, employing the Nijmegen NSC97a-f and NSC89 hyperon-nucleon
potentials.
