Cook the spaghetti or egg noodles according to package instructions until al dente. Drain and set aside.
In a large skillet, heat the vegetable oil over medium heat. Add the minced garlic and red pepper flakes (if using) and sauté for about 1 minute until fragrant.
In a small bowl, whisk together the soy sauce, oyster sauce, brown sugar, and sesame oil. Pour this mixture into the skillet with the garlic and stir to combine.
Add the cooked noodles to the skillet and toss well to coat them in the sauce. Cook for an additional 2-3 minutes, stirring occasionally, until heated through. Remove from heat and set aside.
In another skillet, season the chicken thighs with salt and pepper. Cook over medium-high heat for about 5-7 minutes on each side, or until fully cooked and golden brown.
Once cooked, remove the chicken from the skillet and let it rest for a few minutes. Slice the chicken into strips and drizzle with teriyaki sauce.
Serve the garlic noodles topped with sliced teriyaki chicken and garnish with sliced green onions.