  he bound-state level structures of the $^{16}_{\Lambda}$O and
$^{15}_{\Lambda}$N hypernuclei were studied by $\gamma$-ray spectroscopy using
a germanium detector array (Hyperball) via the $^{16}$O ($K^-, \pi^- \gamma$)
reaction. A level scheme for $^{16}_{\Lambda}$O was determined from the
observation of three $\gamma$-ray transitions from the doublet of states
($2^-$,$1^-$) at $\sim 6.7$ MeV to the ground-state doublet ($1^-$,$0^-$). The
$^{15}_{\Lambda}$N hypernuclei were produced via proton emission from unbound
states in $^{16}_{\Lambda}$O . Three $\gamma$ -rays were observed and the
lifetime of the $1/2^+;1$ state in $^{15}_{\Lambda}$N was measured by the
Doppler shift attenuation method. By comparing the experimental results with
shell-model calculations, the spin-dependence of the $\Lambda N$ interaction is
discussed. In particular, the measured $^{16}_{\Lambda}$O ground-state doublet
spacing of 26.4 $\pm$ 1.6 $\pm$ 0.5 keV determines a small but nonzero strength
of the $\Lambda N$ tensor interaction.
