Second-order rate constants have been measured spectrophotometrically for the reactions of phenyl Y-substituted-phenyl carbonates 7a-g with butane-2,3-dione monoximate ($Ox^-$) in 80 mol % $H_2O$/20 mol % DMSO at $25.0{pm}0.1^{circ}C$. The ${alpha}$-nucleophile $Ox^-$ is 53-95 times more reactive than the corresponding normal-nucleophile 4-$ClPhO^-$ toward 7a-g, indicating that the ${alpha}$-effect is operative. The magnitude of the ${alpha}$-effect (e.g., the $k_{Ox^-}/k_{4-ClPhO^-}$ ratio) is independent of the electronic nature of the substituent Y. The cause of the ${alpha}$-effect for the reactions of 7a-g has been suggested to be ground-state (GS) effect rather than transition-state (TS) stabilization through a six-membered cyclic TS, in which $Ox^-$ behaves a general acid/base catalyst. This idea is further supported by the result that $OH^-$ exhibits negative deviation from the linear Br${o}$nsted-type plot composed of a series of aryloxides, while $Ox^-$ deviates positively from the linearity. Differential solvation of the GS of $Ox^-$ and 4-$ClPhO^-$ has been suggested to be responsible for the ${alpha}$-effect exerted by $Ox^-$.