Turn slow cooker on sauté mode if that's available otherwise use a heavy pot. Add oil to heat. Season oxtails with salt and pepper and brown on all sides (8-12 minutes). Set aside
To the same pot or slow cooker on sauté mode add in onion and garlic and cook until soft and fragrant. Season with oregano and fresh thyme.
Add tomato paste and wine. Stir and let wine reduce by half (about 4-6 minutes).
Transfer oxtails back to the slow cooker. If using a pot to sauté you can transfer the onion in wine sauce and oxtails to the slow cooker.
Add broth and bay leaves. Cover and set timer for 4 hours on high or 8 hours on low heat.
