Rinse pork tenderloin and pat dry. Drizzle with olive oil and rub with oregano. Set aside.
Layer slow cooker with onions, tomatoes, green chilies, garlic, chili powder, cumin and beer. Cook on low until pork is tender and easily pulls apart.
Remove the pork and pull it apart with 2 forks.
Before adding pork back to slow cooker, add the tomato paste to thicken sauce in the slower cooker. Return pork to slow cooker for 30 minutes.
Season with salt and pepper to taste. Just before serving add a squeeze of lime juice and sprinkle with cilantro.