NailO is a nail-mounted gestural input surface inspired by commercial nail stickers. Using capacitive sensing on printed electrodes, the interface can distinguish on-nail finger swipe gestures with high accuracy (>92 percent). NailO works in real time: the system is miniaturized to fit on the fingernail, while wirelessly transmitting the sensor data to a mobile phone or PC. NailO allows for one-handed and always-available input, while being unobtrusive and discrete. The device blends into the user's body, is customizable, fashionable, and even removable.