Mixtures of MgB₂ and metal or oxide additions with starting compositions of (MgB₂)(M₂O₃) _x , x = 0.0025, 0.005, 0.015, and (MgB₂)(M) _y , y = 0.01, M = Bi, Sb, were processed by spark plasma sintering (SPS). As-obtained samples are composites with high density exceeding 94% of the theoretical values. Secondary phases indicate similar reactions for samples with Bi- or Sb-based additions. However, samples show very different superconducting characteristics depending on the addition type and amount. A direct correlation with the melting temperature of the addition could not be revealed, although some aspects will be discussed. From the critical current density (J _c) and irreversibility field (H _irr) enhancement viewpoints, optimum additions are oxides for x=0.0025, 0.005. Both oxides improve J _c at high fields, but Sb₂O₃ is effective up to 10 K, whereas Bi₂O₃ is effective up to 30 K. Metal additions decrease J _c and H _irr when compared with a pristine MgB₂ sample.