Heat 2 tablespoons of olive oil in a large skillet over medium heat until hot. Add onion; cook for 3 minutes or until translucent. Add garlic; cook for 1 minute. Add ground beef; season with the oregano, salt and pepper then cook, breaking up with a wooden spoon, until browned. Remove from pan and set aside.
Heat remaining 2 tablespoons of oil in the skillet. Add zucchini, yellow squash and tomatoes; season to taste with salt and pepper then cook and stir for 8 minutes or until vegetables start to soften. Return ground beef to the pan; stir to combine. Continue to cook for 5 minutes, to allow the flavors to combine. Check for seasoning and serve.
*To de-seed a tomato, simply cut in half and squeeze the seeds out over a bowl or your sink. You can do so over a sieve on top of a bowl in order to separate the seeds from the juice. You can then add the juice to the recipe even if no liquid is called for.