  The photon-ion merged-beams technique was used at a synchrotron light source
for measuring absolute cross sections of double and triple photodetachment of
O$^{-}$ ions. The experimental photon energy range of 524-543 eV comprised the
threshold for K-shell ionization. Using resolving powers of up to 13,000, the
position, strength and width of the below-threshold 1s 2s2 2p6 2S resonance as
well as the positions of the 1s 2s2 2p5 3P and 1s 2s2 2p5 1P thresholds for
K-shell ionization were determined with high precision. In addition,
systematically enlarged multi-configuration Dirac-Fock calculations have been
performed for the resonant detachment cross sections. Results from these
ab-initio computations agree very well with the measurements for the widths and
branching fractions for double and triple detachment, if double shake-up (and
-down) of the valence electrons and the rearrangement of the electron density
is taken into account. For the absolute cross sections, however, a previously
found discrepancy between measurements and theory is confirmed.
