Rub the pork shoulder with ground cumin, dried oregano, sea salt, and black pepper until evenly coated.
Place the seasoned pork in the slow cooker and pour in the orange juice and lime juice, then add the smashed garlic cloves.
Cover and cook on low for 8 hours or on high for 4-5 hours until the meat is tender and easily shredded with a fork.
Remove the pork from the slow cooker and shred it using two forks, discarding any large pieces of excess fat.
Heat a large non-stick skillet over medium-high heat and add the shredded pork in a single even layer.
Press down with a spatula and sear for 3-4 minutes until the edges are golden and crispy using the pork's natural rendered oils.
Warm the corn tortillas in a dry pan or over an open flame for 30 seconds per side until pliable and slightly charred.
Divide the crispy pork between the tortillas and top with diced red onion, fresh cilantro, and thinly sliced radishes.