In a large skillet, heat olive oil over medium-high heat.

Add the oxtail pieces and sear them until browned on all sides, about 5-7 minutes.

Transfer the browned oxtail to the slow cooker.

In the same skillet, add the chopped onion, garlic, carrots, and celery; sauté for 3-4 minutes until the vegetables start to soften.

Add the sautéed vegetables to the slow cooker with the oxtail.

Stir in the thyme, oregano, bay leaf, beef broth, diced tomatoes, tomato paste, salt, black pepper, and red pepper flakes (if using).

Cover and cook on low for 8 hours or on high for 4 hours, until the oxtail is tender.

About 30 minutes before serving, add the frozen peas to the slow cooker.

Once cooked, remove the oxtail pieces, shred the meat off the bones, and return the meat to the soup, discarding the bones and bay leaf.

Serve hot, garnished with fresh parsley.