Combine water and lentils in a pot and bring to a boil; cook at a boil until the lentils are tender but not mushy, 15 to 20 minutes. Drain and rinse with cold water to cool.
Gently toss lentils, celery, and red onion together in a large bowl. Whisk balsamic vinegar, white vinegar, olive oil, and oregano together in separate bowl; drizzle over lentil mixture and toss to coat.
Cover bowl with plastic wrap and refrigerate at least 1 hour, up to 2 days.