Skewers: Thread skewers starting with a tomato, then a piece of feta cheese, an olive, a piece of onion, and finish with another tomato.
Vinaigrette: In a small bowl, mix together the lemon juice, red wine vinegar, and 2 teaspoons of the oregano. Whisk in olive oil until mixture thickens. Season with salt and pepper to taste.
Arrange skewers on a serving platter. Spoon vinaigrette over skewers and sprinkle with remaining oregano.