Roughly chop Swiss chard leaves and stems, keeping them separate. Heat oil in a large skillet over medium heat and add onion, chard stems and garlic to the pan. Cook for 3 minutes or until slightly softened. Add squash, chard leaves, oregano , red pepper flakes and salt and pepper to the pan. Toss the mixture until all of the veggies are coated with the oil. Continue to cook until the Swiss chard has wilted and the squash are cooked through, stirring occasionally. About 10 minutes. Remove the veggies from the pan and toss with crumbled feta cheese. Serve immediately.