  Here we describe the results of an atomic resolution study of oxygen
incorporation into bulk MgB2. We find that ~20-100 nm sized precipitates are
formed by ordered substitution of oxygen atoms onto boron lattice sites, while
the basic bulk MgB2 crystal structure and orientation is preserved. The
periodicity of the oxygen ordering is dictated by the oxygen concentration in
the precipitates and primarily occurs in the (010) plane. The presence of these
precipitates correlates well with an improved critical current density and
superconducting transition behavior, implying that they act as pinning centers.
