Heat oil in a skillet over medium-high heat. Add onion and cook until softened, about 4-5 minutes.
Add garlic and cook until fragrant, stirring occasionally, about 30 seconds. Transfer onion mixture out of pan and place in slow cooker.
Add remaining ingredients to slow cooker and stir to combine. Cook for 3-4 hours on high or 6-8 hours on low.
To serve, transfer chicken out of slow cooker and shred with two forks. Return to slow cooker and stir to combine.
