There are a couple of ways to do this that I know of. Both would be described as diffusion. The first one sound more suitable for you because you have already fixed the location of your LED(s) with respect to the enclosure. You would look to use a translucent piece of plastic with the LED set back some distance from it. Or you could use transparent plastic which has had the surface roughened, perhaps with wet and dry paper or fine sandpaper. Wet sanding can result in a more even finish in my experience. Another way you could implement your “O” (I would describe it as a ring doughnut or annular shape) might be instead of cutting a hole in your opaque enclosure face, installing a ring and then installing an opaque circle to fill the centre, you could make the whole lid out of translucent plastic and add light-blocking film or paint over the top, like Adam suggests.
The second approach is edge lighting. This is where you get a transparent material such as acrylic sheet, which is thick enough to shine the LEDs into the edges of the sheet and achieve total internal reflection. You then roughen the large surface of the transparent sheet and light spills out there, because the internal reflection fails. But you need to be able to place your LED at the edge of the sheet to make this work and it might not be an option for you with this project.
Congratulations on your first PCB.