Rinse the pork shoulder and pat dry with a paper towel. Transfer to a large cutting board and generously season with salt and black pepper.
In a small bowl, combine the oregano, cumin, chili powder, and olive oil. Whisk to form a paste.
Using your hands, spread the 'wet rub' onto the roast and rub the spices into the meat.
Transfer the pork to the dish of a slow cooker. Add the diced onion, garlic, and jalapeño. Squeeze the fresh orange juice over the roast before tossing the orange halves into the slow cooker.
Cook:
Cover and set to cook for 4 hours on high or 10 hours on low.
Shred the meat:
Once the meat is tender and pulls apart easily, remove the roast onto the cutting board and shred the meat between 2 forks.
Make 'em crispy:
In a large skillet, heat the vegetable oil over medium-high heat. Working in batches, place the shredded pork in an even layer in the skillet. Cook until the edges develop a brown crust.
Warm up the tostadas:
Toast the tostadas in a toaster oven or in the oven at 350F for 5 minutes.
Serve:
Top the crunchy tostadas with carnitas, cheese, salsa, guacamole, and sour cream.