Melt butter in a large skillet over medium heat. Cook pork belly until crispy but still soft in some spots, stirring often with a wooden spoon, about 6 minutes. Drain on a paper towel; discard grease.
Return pork belly to the hot skillet. Add water and oyster sauce; cook and stir until combined, about 2 minutes. Toss in spinach; increase heat to medium-high. Stir until fully coated with sauce. Cover and cook, stirring occasionally, until wilted, about 5 minutes.