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.
Serve hot.
Video
Notes
Cut the upo in uniform size to ensure even cooking.