Cook 2 cups of sushi rice according to package instructions and let it cool slightly.
Drizzle rice vinegar in the rice and mix well.
In a bowl, whisk together the soy sauce, oyster sauce, and brown sugar until combined. Set aside.
Crack 7–8 eggs into a pan and cook into a thin, even omelet. You may need to cook in 2 batches if your pan is not large enough. Set aside and cut to fit your musubi.
Slice the Spam into 8 equal pieces.
Heat a pan over medium heat and sear the Spam slices on both sides until lightly browned.
Reduce heat to low and pour in the sauce. Cook, flipping the Spam, until the sauce thickens and lightly caramelizes on both sides. Remove from heat.
Cut the nori sheets into thirds.
Using a Spam musubi mold (or an empty Spam can lined with plastic wrap), place a strip of nori inside the can. Add a layer of rice, followed by egg, then a slice of Spam. Press gently to compact.
Wrap the nori around the musubi. Use a few grains of rice along the edge to help seal if needed.
Serve warm and enjoy.