Yes, belts are indeed allowed in Olympic weightlifting, and there aren’t any rules against them in the slightest. In fact, weightlifting belts are simply considered a standard piece of gear within weightlifting, and can therefore be used without causing any issues.
Not every weightlifter will use a belt, but it is quite common to see most of them wearing a belt, as it provides health benefits during the harder lifts. It is especially common to see weightlifters using a belt during their single lift, as it increases IAP.