Heat olive oil over medium heat. Add onion and cook, stirring occasionally, until soft, about 5 minutes. Add spinach, garlic and oregano; cook and stir until the spinach is wilted, abut 3 minutes. Remove from heat.
Cut the potato in half and scoop the insides into a bowl. Place the skins on a baking dish or 8x8 inch pan.
Add cream cheese, pepper, salt and about half the feta to the bowl. Beat with a hand mixer or mash with a potato masher until desired smoothness. I like a little chunky. Stir in the spinach mixture.
Stuff each potato skin half the filling. Top each with remaining feta.
Bake until the filling is steaming and the feta is browned, about 30 minutes.