Heat oil in a large pan over medium-high heat.
Add cubed pork and cook until browned on all sides.
Remove pork and set aside.
In the same pan, sauté onions and garlic until fragrant.
Add bell peppers and cook for 3-4 minutes.
Stir in pineapple chunks and cook for an additional 2 minutes.
Mix ketchup, vinegar, brown sugar, and soy sauce in a bowl.
Pour sauce into the pan and bring to a simmer.
Return pork to the pan and stir to combine.
Add cornstarch mixture to thicken the sauce and cook for another 2-3 minutes.