Rinse jalapeños and pierce each with a fork. Peel carrots and cut crosswise into 1/4-inch slices. Peel onion and cut vertically into 1/4-inch slivers.
In a 3- to 4-quart pan over high heat, combine jalapeños, carrots, onion, 2 cups water, vinegar, oil, garlic, oregano, and bay leaf. Bring to a boil.
Reduce heat and simmer just until carrots are tender-crisp when pierced, 5 to 8 minutes. Add salt and pepper to taste.
Pour into jars, cover, and refrigerate at least 1 day. Use a slotted spoon to serve.