In a wide pan or wok over medium heat, heat oil.
Add onions and garlic and cook until softened.
Add pork and cook, occasionally stirring, until lightly browned.
Add tomatoes and cook, mashing with the back of the spoon until softened.
Add water and bring to a boil. Lower heat, cover, and simmer until meat is tender and liquid is reduced to about 1/2 cup.
Increase heat to medium. Add upo and cook, uncovered, for about 3 to 5 minutes or until half done.
Add oyster sauce and stir to distribute. Season with salt and pepper to taste.
Continue to cook, stirring occasionally for about 2 to 4 minutes or upo is tender yet crisp. Serve immediately.