Heat oil in a large skillet over medium-high heat. Add onions and cook, stirring occasionally, until they begin to soften, 3 to 6 minutes. Add sugar and continue to cook, stirring constantly, until the onions are golden brown, 6 to 8 minutes more. Add garlic, oregano, pepper and salt and cook, stirring, for 1 minute. Add vinegar and bring to a boil. Cook until mostly evaporated, 30 seconds to 1 minute. Remove from the heat and stir in chili sauce and chipotle to taste.

Place pork in a 4-quart (or larger) slow cooker and cover with the sauce. Cover and cook until the pork is almost falling apart, about 4 hours on High or 8 hours on Low.

Transfer the pork to a cutting board and shred using two forks. Stir back into the sauce.