Preheat the oven to 375 degrees. Season the osso buco with 1/2 teaspoon salt and 1/4 teaspoon pepper, and dredge them in flour. Heat the olive oil in a 12-inch skillet or dutch oven over medium high heat. Brown the osso buco on all sides, about 15-20 minutes. Transfer the osso buco to a platter and reserve.
Pour all the fat from the pan. Add the vinegar, and with a wooden spoon, scrape up any brown bits adhering to the pan. Cook over high heat until the vinegar is reduced by half. Add the veal stock to the pan and bring to a simmer. Season with the remaining salt and pepper.
Return the osso buco to the pan. Cover and braise in the oven for 1 hour.
Split the fennel in half lengthwise leaving the core intact. Slice each half lengthwise into 1/2-inch pieces.
Uncover the osso buco and add the fennel, carrots and orange juice. Raise the oven temperature to 400 degrees and cook uncovered an additional 45-50 minutes, basting occasionally. The meat should be fork-tender and the vegetables completely cooked.
Remove the pan from the oven. Transfer the osso buco to a serving platter and keep warm. Skim the fat from the cooking liquid with a bulb-baster or ladle, and cook over high heat to thicken slightly, 4-5 minutes. Stir in orange segments, ladle the sauce and vegetables over the meat and serve.