Catalyzes the conversion of O-acetyl-L-homoserine (OAH) into homocysteine in the methionine biosynthesis pathway (PubMed:11754480, PubMed:6526818). Can also use O-succinyl-L-homoserine and L-homoserine as substrates (PubMed:6526818). Has also cysteine synthase (O-acetylserine sulfhydrylase) activity in vitro, but in S.pombe, it seems only to be involved in the alternative pathway of methionine biosynthesis under cysteine deficiency conditions (PubMed:11754480). methanethiol + O-acetyl-L-homoserine = acetate + H(+) + L-methionine Optimum pH is 8. Amino-acid biosynthesis; L-methionine biosynthesis via de novo pathway; L-homocysteine from O-acetyl-L-homoserine. Homotetramer. Repressed by cysteine. Belongs to the trans-sulfuration enzymes family.