In a small bowl, make the spice rub: stir together chili powder, cumin, oregano, salt and pepper.
Rub the spice mix over the pork, covering all sides.
Place the pork in the slow cooker. Add orange juice and lime juice. Add onion and garlic on top of the pork.
Cover and cook on low for 8 - 10 hours or high for 5 - 6 hours. The pork should be cooked through and tender enough to easily shred.
Remove the pork from the slow cooker and place on a cutting board. Let cool for 5 - 10 minutes and then shred using two forks.
Add the shredded pork to a baking sheet in 1 layer and pour over 1 cup of liquid from the slow cooker.
Place the pork in the oven and turn on to broil at 450 degrees. Broil 7 - 10 minutes, until the edges of the pork are darkened and crispy.
To serve: if desired, pour additional liquid from the slow cooker over the pork before serving. You can eat these carnitas alone or in tacos, burritos, nachos, etc!