Preheat the oven to 425ºF. Set aside a baking sheet.
Core the tomatoes and slice them across into slices 1/4 – 1/2-inch thick. Arrange in a single layer on the baking sheet. Drizzle with the olive oil and sprinkle on salt, pepper, and the oregano. Bake in the preheated oven for 10 minutes.
Remove from the oven and spread each tomato with a spoonful of the pesto and sprinkle with the Parmesan cheese. Return to the oven and cook until the Parmesan is melted and starts to brown, 7 to 10 more minutes.