Heat olive oil in large skillet; add onion, garlic, and ginger; saute until fragrant, about 2-3 minutes. Add chicken and brown on both sides. Transfer to slow cooker. Add curry powder to skillet and stir a minute or two until fragrant. Add chicken broth to skillet and scrape up brown bits. Transfer to slow cooker. Add oyster sauce, rice wine, ketchup and sugar to slow cooker and toss until chicken is coated with sauce. Cover and cook on high 4 hours. Stir in scallions before serving.