Heat oil in heavy large Dutch oven over high heat. Season oxtails with salt and pepper. Add to Dutch oven and cook until brown on all sides, about 12 minutes. Transfer oxtails to platter. Add celery, carrots, leeks, onion and garlic to pot and saut, until golden brown, about 8 minutes. Arrange oxtails over vegetables. Add wine, thyme, parsley, cloves and bay leaves. Boil until liquid is reduced by half, about 15 minutes. Add stock. Bring to boil. Reduce heat. Cover Dutch oven tightly and simmer until meat is very tender, about 1 1/2 hours.

Remove oxtails from cooking liquid. Tent with foil to keep warm. Strain cooking liquid. Skim fat from surface and reserve. Transfer 2 tablespoons skimmed fat to heavy medium saucepan over medium heat. Add flour and stir until mixture browns, about 10 minutes. Whisk in cooking liquid. Simmer until sauce thickens enough to coat back of spoon and is reduced to 2 cups, stirring frequently, about 45 minutes. Season to taste with salt and pepper. (Can be prepared 1 day ahead. Return oxtails to sauce, cover and refrigerate.)

Rewarm oxtails in sauce over low heat. Transfer to large bowl and serve.