Preheat the oven to 425 degrees F (220 degrees C). Lightly oil a baking pan large enough to hold oxtails in a single layer.

Rub oxtail sections with olive oil; season with salt and black pepper. Place onion and garlic in prepared pan; place oxtails on top.

Roast in the preheated oven 25 minutes. Flip oxtails; continue roasting until well browned, 20 to 35 minutes more.

Add tomato sauce, chicken stock, vinegar, thyme, oregano, rosemary, 1 teaspoon kosher salt, and red pepper flakes to the baking pan; stir and transfer mixture to a large pot. Place over medium-high heat until sauce begins to simmer. Cover, reduce heat, and simmer over low heat until oxtails very tender and easily separate from bones, 4 to 5 hours. Skim off fat before serving.