Place pork belly in a large pot and pour in enough water to cover pork by 1 inch; bring to a boil. Reduce heat to medium and simmer for 2 hours. Discard water and fill pot with fresh water 1-inch above the pork. Bring water to a boil, reduce heat to medium, and simmer until pork begins to soften, about 1 hour.
Remove pork from the water and place on a work surface. Allow pork to cool for a few minutes.
Slice the thick skin off the pork and discard. Cut the pork meat into 1-inch-wide slices.
Combine soy sauce, brown sugar, oyster sauce, mirin, 1/2 cup water, ginger, and 1 large and 1 small clove garlic in a large saucepan; bring to a boil. Add the pork and bring to a boil; reduce heat to low and place a sheet of aluminum foil directly over pork and sauce. Simmer mixture, turning pork frequently, until evenly cooked, 30 to 45 minutes.