Abstract: In order to study the importance of carboxypeptidase in protein hydrolysis and debittering, a truncated carboxypetidase O from Asperigillus oryzae was expressed in Pichia pastoris. The recombinant carboxypeptidase O (rOcpO) was deglycosylated and characterized after purified through Sephadex G-75 and DEAE anionic-exchange chromatography. The results showed that rOcpO was glycosylated of a molecular weight of about 74 ku and 56 ku after been deglycosylated, the enzyme yield was about 20.4 nKat/mL. rOcpO was an acid protease, its optimal pH was about 4.0, and it was stable in the pH range of 3-6; its optimal temperature was about 40益, it retained 63% enzyme activity after 1 h incubation at 60益; PMSF could efficiently inhibit its activity, which confirmed it was a serine-type carboxypeptidase. The bitterness of SPI and casein hydrolysates catalyzed by Alcalase significantly reduced after treated with rOcpO and the degree of hydrolysis (DH) efficiently improved at the same time.