The H + OCS potential-energy surface (PES) was used to evaluate the performance of density functional theory by comparing the results to ab initio calculations at the QCISD(T)//UMP2 and UMP2 levels using the aug-cc-pVTZ and 6-311+G(2df, 2p) basis sets. The two major reaction paths on this PES involve formation of OH((II)-I-2) + CS((1)Sigma) (reaction I) and SH((II)-I-2) + CO((1)Sigma) (reaction II). Experimental and QCISD(T)//UMP2/aug-cc-pVTZ activation barriers for (II) and reaction enthalpies for (I) and (II) were compared to values calculated by several density functionals (BLYP, B3LYP, B3PW91, BPW91, BP86, and B3P86) using the aug-cc-pVTZ basis set. All DFT/aug-cc-pVTZ predictions, except for the B3LYP prediction of the enthalpy of reaction I, were outside the range of experimental uncertainty. B3LYP predictions were in closest agreement with the experimental values and QCISD(T) predictions. B3LYP, BPW91, and B3PW91 predictions of the rate-limiting barrier to reaction II are within 3.5 kcal/mol of the QCISD(T) prediction, and all DFT values are below that of the QCISD(T). Reaction enthalpies for (I) and (II) were calculated using the BHandHLYP density functional and the 6-311+G(2df,2p) basis set. These predictions were closer to experiment and QCISD(T) values than any other DFT calculations, and the predicted enthalpy for reaction I is within the range of experimental values. The second portion of the study compared B3LYP and BLYP predictions of the 12 transition states and 6 stable intermediates within this PES with previously reported QCISD(T)//UMP2/6311+G(2df,2p) predictions. The complexity of this surface allows for the evaluation of barrier heights for 28 reactions involving hydrogen addition, elimination, isomerization, migration, and radical diatomic elimination. With the exception of five reactions, all B3LYP barrier heights are within 3.7 kcal/mol of the QCISD(T) predictions and in several cases are in as good or better agreement than the UMP2 predictions. In addition, all but one of the B3LYP barriers lie below the QCISD(T) values. The most significant differences between the ab initio and DFT predictions were in the saddle points for radical elimination or addition. BLYP/6-311+G(2df, 2p) failed to find the two transition states associated with SH elimination from the cis- and trans-HSCO species. B3LYP located the saddle point for SH elimination from cis-HSCO, but its prediction of a saddle-point structure for SH elimination from trans-HSCO has an energy (without zero-point corrections) lower than that of the products. These transition states were subsequently optimized using the BHandHLYP functional and the 6-311+G(2df,2p) and 6-31G** basis sets. The geometries of these saddle points were in better agreement with UMP2/6-311+G(2df,2p) predictions than were the BLYP and B3LYP predictions. The BLYP predictions are in overall worse agreement with the QCISD(T) results than are the B3LYP predictions.