Rinse Swiss chard with cold water; drain chard well.
Heat oil in a large nonstick skillet over medium-high heat. Add garlic, and sauté 1 minute or until slightly golden. Add chard. Cover and cook 1 minute or until chard begins to wilt. Stir in oregano, salt, and pepper. Cover and cook 5 minutes or until tender, stirring occasionally. Remove from heat; stir in vinegar.