Heat a heavy-bottomed pot over medium heat. Add the olive oil and pancetta; cook for 3 minutes; and then add the carrots, celery, onion, oregano, and chile flakes. Cook, stirring now and then, until soft and lightly browned, about 12 minutes.
Turn down the heat if the vegetables start to brown too quickly.
When the vegetables are cooked, add the salt, garlic, and cooked chickpeas. Cook for a few minutes, and then pour in the chickpea cooking liquid and chicken stock. Bring to a boil and reduce to a simmer. Cook for 10 minutes.
Meanwhile, trim off and discard the woody stems from the broccoli rabe. Wash and drain, chop coarsely, and add to the soup. Cook for another 10 minutes. Test a large rabe stem. If it is not tender, cook the soup a few more minutes. Taste for salt and adjust as needed.