In a bowl, whisk together the marinade ingredients: soy sauce, water, sugar, minced garlic, oyster sauce, sesame oil, and black pepper. Stir until the sugar is dissolved.

Place the chicken pieces into a shallow dish or a ziplock bag, and pour the marinade over the chicken, making sure all the pieces are coated. Marinate the chicken in the refrigerator for at least 2 hours, or overnight for best results.

When ready to cook, remove the chicken from the marinade and drain off the excess liquid. Discard the marinade.

Heat about 2 inches of vegetable oil in a deep frying pan or wok over medium-high heat, to a temperature of around 350°F (175°C).

Place the flour or cornstarch in a shallow dish. Dredge each chicken piece in the flour or cornstarch, shaking off any excess.

Carefully add the coated chicken pieces into the hot oil, in batches if necessary to avoid overcrowding the pan. Fry the chicken for about 5-7 minutes, turning occasionally, until they are golden brown and cooked through.

Use a slotted spoon or tongs to remove the fried chicken from the oil and transfer to a paper towel-lined plate to drain any excess oil.

Repeat the frying process with the remaining chicken pieces until all are cooked.