Deep-ultraviolet (DUV) materials with giant optical birefringence and nonlinearity are scarce, and their discovery is very challenging. In this Communication, we predict that the MCO [M = S, (Cl2B)(3)B] compounds containing the polar C-O motif, which have been synthesized in the experiments, could be the first material systems to achieve this rare capability. First-principles calculations demonstrate that (Cl-3)(3)BCO and SCO exhibit the largest second-harmonic-generation effect (similar to 4.4 pm/V) and the largest birefringence (similar to 0.6) in all known DUV nonlinear-optical (NLO) crystals. In addition, SCO might be a DUV NLO material for the practical 193.7 nm laser output. Our discovery could enrich the structural chemistry of NLO crystals and advance the development of functional DUV optical materials.