In a bowl, stir together the tomatillo salsa verde, dried oregano, and diced green chilies until combined.
Season the chicken thighs with salt and pepper on both sides.
Pour the salsa mixture into the bottom of the slow cooker and arrange the seasoned chicken thighs in the sauce.
Cover and cook on low for 4–5 hours, or until the chicken is tender and easily shreds with forks.
Remove the chicken to a cutting board and shred with two forks or cut into chunks; discard any excess liquid with a slotted spoon if desired.
Warm the flour tortillas in a skillet or tortilla warmer, then fill each tortilla with about 1/4 cup of the shredded chicken.
Top tacos with diced avocado, chopped cilantro, and a squeeze of fresh lime juice, if using, and serve immediately.