Which one is better leaving group H2O or OH?

H₂O is generally a better leaving group than OH⁻. This is because H₂O is neutral and more stable in solution, while OH⁻ carries a negative charge, making it less stable and more reactive. As a leaving group, stability is key; a good leaving group should be able to stabilize the negative charge after leaving, and H₂O fulfills this requirement more effectively than OH⁻.