A set-top box, which allows a user to input a key command in real time while displaying an animation sequence on an on-screen display (OSD) screen, and a method of outputting the OSD data of the set-top box is provided. The set-top box includes OSD control module that divides OSD animation data into a plurality of subsets, loads each of the subsets in every operating cycle, determines whether a user has input a key command in every operating cycle, and selectively provides the loaded subsets based on the determination result in every operating cycle, wherein the OSD animation data realizes an animation sequence when displayed. The set-top box further includes a combination control module that receives the loaded subsets of the OSD animation data from the OSD control module and overlays the data onto the decoded broadcast signal and provides the overlaid result to a display device.