Bring 1 1/4 cups lightly salted water up to a boil. Add couscous, stir and reduce heat to medium.
Cook uncovered, stirring occasionally, until all liquid has absorbed. Scrape couscous into a strainer and rinse under cold water to cool.
Place into a bowl and toss with olive oil to coat. Add rocket, olives, onion, celery, lemon juice and feta and toss. Season to taste and chill until ready to serve.
Couscous can be prepared up to 8 hours in advance.