Put the sliced onion in a bowl of cold water and leave to soak while you prepare the remaining ingredients. Carve the watermelon lengthways into three wedges, then cut away the skin and slice into 1½ cm-thick pieces. Halve the cucumber lengthways and scoop out the seeds with a teaspoon, then slice into half-moons about ½ cm thick.
Arrange the watermelon and cucumber on 4 plates, then scatter over the olives and feta. Drain the onion and add to the salad. Drizzle each plate with 1 tbsp of oil and 1 tsp of vinegar; season. Serve immediately, sprinkled with the mint and oregano.