Cook orzo following package directions. When almost done, stir in peas. Meanwhile, heat olive oil in a large nonstick skillet. Add onion and garlic; cook for 3 minutes over medium-high heat, stirring occasionally. Stir in marinara sauce, dried oregano and black pepper. Simmer for 3 minutes. Add shrimp and cook for 6 minutes, stirring occasionally. Stir in lemon juice. Spoon shrimp and sauce over the orzo. Crumble cheese over the top. Garnish with lemon slices.