Add the lentils, onion, carrots, garlic, vegetable broth, and oregano to the slow cooker. Stir to combine.
Cover and cook on low for 6 to 7 hours until the lentils are tender.
While the stew cooks, boil the orzo in a separate pot according to package directions. Drain and set aside.
Stir the chopped kale into the slow cooker during the last 20 minutes of cooking so it softens.
Turn off the heat. Add the lemon zest, lemon juice, salt, and pepper. Stir in the cooked orzo.