  Absolute cross sections for the {\it K}-shell photoionization of boron-like
(B-like) O$^{3+}$ ions were measured by employing the ion-photon merged-beam
technique at the SOLEIL synchrotron-radiation facility in Saint-Aubin, France.
High-resolution spectroscopy with E/$\Delta$E $\approx$ 5000 ($\approx$ 110
meV, FWHM) was achieved with photon energy from 540 eV up to 600 eV. Several
theoretical approaches, including R-Matrix, Multi-Configuration Dirac-Fock and
Screening Constant by Unit Nuclear Charge were used to identify and
characterize the strong 1s $\rightarrow$ 2p and the weaker 1s $\rightarrow$ 3p
resonances observed in the {\it K}-shell spectra of this ion. The trend of the
integrated oscillator strength and autoionisation width (natural line width) of
the strong $\rm 1s \rightarrow 2p$ resonances along the first few ions of the
B-like sequence is discussed.
