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 annatto powder and stir to coat meat.
Add water and bring to a boil. Lower heat, cover, and simmer until meat is tender and liquid is reduced to about 1 cup.
Increase heat to medium. Add upo and cook for about 1 to 2 minutes or until half-done.
Add noodles and cook, stirring to separate.
Add oyster sauce and stir to distribute. Season with salt and pepper to taste.
Continue to cook, stirring regularly, for about 3 to 5 minutes or until noodles cooked through, upo is tender yet crisp, and liquid is mostly absorbed.