Peel and segment the citrus. Using a paring knife, slice a thin slice off the top and the bottom of the fruit, so that it has a stable base. Then, start to slice off the peel in strips, moving your knife from top to bottom, following the natural curve of the fruit. Repeat, moving around the perimeter of the fruit, until there's no more peel and the flesh is visable.
To remove the segments, hold the fruit in the palm of your hand and then slice between the membranes toward the center of the fruit to release a pretty little segment of citrus. Repeat for each fruit.
Arrange the orange and grapefruit segments in a shallow dish. Sprinkle the olives over the fruit. Drizzle the oil over the citrus segments and sprinkle with the coriander, salt and pepper. Serve.