Catalyzes the conversion of O-acetylserine (OAS) to cysteine through the elimination of acetate and addition of hydrogen sulfide. hydrogen sulfide + O-acetyl-L-serine = acetate + L-cysteine Amino-acid biosynthesis; L-cysteine biosynthesis; L-cysteine from L-serine: step 2/2. Homodimer. Belongs to the cysteine synthase/cystathionine beta-synthase family.