Powered lower-limb orthosis is a type of wearable mechanical devices that can serve a wide variety of important biomedical purposes. Due to the constraints associated with the actuation technology, the majority of current lower-limb orthoses are either passive or tethered to external power sources, limiting the functionality of such devices. In this paper, the authors present their preliminary research results towards a fully mobile (i.e. untethered) powered lower-limb orthosis, leveraging the high power density of pneumatic actuators for the joint power generation. The design of the orthosis is presented, with the objectives of providing full assistance in the locomotion of various common locomotive modes, and generating minimum level of restriction to the wearer's daily activities. To regulate the power delivery on the joints for a natural gait assistance, a finite-state impedance controller is developed, which simulates an artificial impedance to enable an effective interaction with the wearer. Preliminary testing demonstrated that the orthosis was able to provide a natural gait and comfortable user experience in the treadmill walking experiments.