Plays an essential role in chain termination during de novo fatty acid synthesis. Possesses high thioesterase activity for oleoyl-ACP versus other acyl-ACPs. Substrate preference is 18:1 > 18:0 > 16:1. (9Z)-octadecenoyl-[ACP] + H2O = (9Z)-octadecenoate + H(+) + holo-[ACP] The catalytic efficiency for 18:1 is at least 20-fold higher than for other substrates. Belongs to the acyl-ACP thioesterase family.