In a bowl, whisk together the BBQ sauce, apple cider vinegar, brown sugar, garlic powder, and Worcestershire sauce.
In a large skillet over medium-high heat, heat the olive oil. Add the chicken thighs, skin side down, and cook until golden. Flip over and cook for a few minutes more.
Carefully transfer the browned chicken thighs to the bottom of a slow cooker. Pour the sauce over the chicken thighs, covering them fully.
Cook for 3 hours on high, or on low for 4 to 6 hours. Once the chicken has reached an internal temperature of 165 F, remove them from the slow cooker.
Carefully pour the sauce into a small saucepan. Cook the sauce, over medium heat, stirring frequently. Bring to a boil and reduce the heat to a simmer. While the sauce simmers, whisk together the cornstarch and water until thickened. Stir the thickened cornstarch mixture into the sauce. Cook the sauce until thickened, or desired thickness has been achieved. Remove the pan from the heat.
Pour the thickened sauce over the chicken thighs. Serve immediately.