Make the couscous by heating the water in a pot. When it boils, add the couscous and stir with a fork. Cover and remove from heat. After 5 minutes, remove lid and fluff with a fork. Place in a large bowl. Let cool slightly.
Once couscous has cooled, add onions, cucumbers, tomatoes, bell peppers and olives.
To make the dressing, in a separate bowl, whisk olive oil and vinegar together. Add garlic, mustard, salt, pepper and oregano. Stir.
Add dressing to couscous mixture. Toss lightly.
Add parsley and toss lightly again.
When quartering the tomatoes, I usually scoop out the seeds and juicy pulp. This is optional.