In a large bowl, combine the apple cider vinegar, water, vegetable oil, sugar, minced garlic, oregano, salt, and pepper. Whisk until the mixture is well blended.
Add the sliced cucumbers and onions to the bowl. Toss everything together to ensure the vegetables are evenly coated with the dressing.
Cover the bowl with plastic wrap and refrigerate for at least 2 hours. For best results, let it marinate overnight to develop more flavor.