Heat olive oil in a saucepan over medium heat. Cook onion until softened, 5 to 6 minutes. Add salt.
Stir rhubarb, dried cranberries, brown sugar, vinegar, ginger, and cinnamon stick into the saucepan. Cook, stirring occasionally, until rhubarb softens and begins to break down, 6 to 8 minutes. Discard cinnamon stick before serving.