Two mononuclear tetrahedral Co(II) complexes (HNEt3)(2)[Co(L-1)(2)]center dot H2O (1) and (Bu4N)(2)[Co(L-2)(2)]center dot H2O (2) (H2L1 = N,N'-bis(p-toluenesulfonyl) oxamide, H2L 2= N,N'-diphenyloxamide) have been synthesized, and their structures have been characterized by single-crystal X-ray diffraction. Both complexes adopt distorted tetrahedral coordination geometries surrounding the Co(II) center, which is ligated by two doubly deprotonated oxamide ligands oriented perpendicularly to each other. Their axial magnetic anisotropies were revealed by the direct current (dc) magnetic measurements, high-field and high-frequency electron paramagnetic resonance, and theoretical calculations. Both complexes display slow magnetic relaxation in the absence of an applied dc field. Upon the application of the 0.15 T dc field, the quantum tunneling of magnetization is efficiently suppressed. In addition, both complexes display hysteresis loops with different field sweep rates at 1.8 K, which is rarely observed for Co(II) single-ion magnets (SIMs).