  Point defects in diamond are of high interest as candidates for realizing
solid state quantum bits, bioimaging agents, or ultrasensitive electric or
magnetic field sensors. Various artificial diamond synthesis methods should
introduce oxygen contamination in diamond, however, the incorporation of oxygen
into diamond crystal and the nature of oxygen-related point defects are largely
unknown. Oxygen may be potentially interesting as a source of quantum bits or
it may interact with other point defects which are well established solid state
qubits. Here we employ plane-wave supercell calculations within density
functional theory, in order to characterize the electronic and magneto-optical
properties of various oxygen-related defects. Beside the trivial single
interstitial and substitutional oxygen defects we also consider their complexes
with vacancies and hydrogen atoms. We find that oxygen defects are mostly
electrically active and introduce highly correlated orbitals that pose a
challenge for density functional theory modeling. Nevertheless, we are able to
identify the fingerprints of substitutional oxygen defect, the oxygen-vacancy
and oxygen-vacancy-hydrogen complexes in the electron paramagnetic resonance
spectrum. We demonstrate that first principles calculations can predict the
motional averaging of the electron paramagnetic resonance spectrum of defects
that are subject to Jahn-Teller distortion. We show that the high-spin neutral
oxygen-vacancy defect exhibits very fast non-radiative decay from its optical
excited state that might hinder to apply it as a qubit.
