Yes. You may create a username & password for your incoming handler and define which information they may reach remotely. In general use, we allow them to reach to reservation, rooming, passenger and transport lists. And authentication to change room state like such as confirmed, not confirmed or on request etc. They may reach also the notices left by your employees about rooms.
But for the creditors (such as incoming handlers or transport companies) Onesystem has some certain restrictions. It does not matter which level of access rights they have, system will let them to reach for the only services they are responsible handling without sales and financial information.