Brown the ground beef in a skillet over medium heat with the diced onion and minced garlic. Drain any excess fat.
Transfer the beef mixture to the slow cooker. Add the cubed potatoes, diced tomatoes, cumin, chili powder, oregano, salt, and pepper.
Stir everything together, cover, and cook on low for 6 hours or until the potatoes are soft.
About 20 minutes before serving, combine the rice and water in a saucepan. Bring to a boil, then reduce heat, cover, and simmer until the rice is tender.
Fluff the rice with a fork and stir in the chopped cilantro. Serve the picadillo over the cilantro rice.