Hi! I have had success using the MPR121 with 5x5mm copper buttons and 2mm overlay. I also found the proximity detection feature to be helpful as it ties the buttons together into one net. For an array of 12 small 5x5mm buttons I found that this could detect a hand approaching at about 2cm. Overall the device is not extremely sensitive but is effective for small buttons with non-conductive overlays. I have not tried an OLED display, but my guess is that the buttons would not work through the display, maybe try ITO on a clear film on top of the OLED display with "traces" in the shape of buttons.
