In a bowl, combine water, oyster sauce, soy sauce and sugar.
In a non-stick skillet, heat oil over a little medium. Pan-fry tofu both sides until golden brown. Drain and set aside.
Return to skillet. Use remaining oil to stir-fry ginger until light brown at medium high heat. Reduce heat to medium, stir in butternut squash, white pepper powder, tofu and sauce mixture. Combine well and bring to boil. Cover and simmer until squash turns tender, about 20 minutes.
Uncover. Add a little more water if necessary. Stir in green onions.