An on-screen display apparatus for displaying button function information on a display is provided. The on-screen display apparatus includes a pushing interpreter, a power button interpreter and a button function information output device. The pushing interpreter is used for determining whether one of a plurality of buttons of the display is pushed. The power button interpreter is electrically connected to the pushing interpreter for determining whether a power button is pushed when one of the buttons of the display is pushed. The button function information output device is electrically connected to the power button interpreter for outputting the button function information onto the display when one of the buttons of the display is pushed while the power button is not pushed.