Catalyzes the conversion of O-acetyl-L-homoserine (OAH) into homocysteine in the methionine biosynthesis pathway. Also catalyzes the conversion of O-acetylserine (OAS) into cysteine, the last step in the cysteine biosynthesis pathway (PubMed:7765825, PubMed:4609980, PubMed:795806). However, it seems that in S.cerevisiae cysteine biosynthesis occurs exclusively through the cystathionine pathway and not via direct incorporation of sulfur into OAS (PubMed:1732168). It therefore has no metabolic role in cysteine biosynthesis and may only have a regulatory role controlling OAS levels (PubMed:12586406). methanethiol + O-acetyl-L-homoserine = acetate + H(+) + L-methionine hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Optimum pH is 7.8 (for O-acetylhomoserine sulfhydrylase activity in Tris-HCl buffer) and 8.4 (for both O-acetylhomoserine sulfhydrylase and O-acetylserine sulfhydrylase activities in barbital-HCl buffer). Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homocysteine from O-acetyl-L-homoserine. Homotetramer. Belongs to the trans-sulfuration enzymes family.