Slice tomatoes about 1/4-inch thick and arrange on a plate with the sliced onions. Place the sliced chile alongside the tomatoes and sprinkle the cilantro on top and around the plate. Crumble the queso fresco on top.
In a bowl combine the oregano, oil, and vinegar and mix well. Drizzle the vinaigrette on top just before serving. Season with salt and pepper, to taste.