First, mix together the pepper, oyster sauce, sesame oil, and brown sugar in a medium bowl. Stir in the chicken, cover, and marinate at room temperature for one hour.
While the chicken marinates, make the stir fry sauce: in a small bowl, stir together the sweet soy sauce, fish sauce, and sambal oelek.
When the chicken is done marinating, heat the vegetable oil on low heat in a wok or cast iron pan. Add the garlic and cook until soft, for about three minutes. Then, add the red pepper, sugar snap peas, and squash. Turn the heat to medium and cook for about three minutes, just until the veggies are crisp tender. Then, remove the vegetable to a serving platter.
Raise the heat to high and add all the chicken. Stir and cook for about five minutes, or until the chicken is no longer pink inside. Add the stir fry sauce and cook for another minutes. Add the pan contents to the vegetables and serve.