Rinse the pork and pat dry with a paper towel.
Combine the oregano, cumin, cinnamon, chili powder, salt, and pepper. Rub it into the pork.
Put the pork into the slow cooker along with bay leaves, garlic cloves, and sliced onion. Squeeze the orange juice into the slow cooker, and add the orange halves as well. Set on low for 10 hours.
After 10 hours, take the pork out and let it cool slightly before shredding. Let the juice in the crock pot cool completely to separate the juice from the fat. (I like to discard the fat and add the juice to the bowl of shredded carnitas to keep it moist)
Heat a skillet or griddle on medium-high heat and crisp up the shredded carnitas.