In a medium saucepan, combine the vinegar, water, sugar, salt, oregano, black peppercorns, garlic, bay leaf, and jalapeño (if using). Bring the mixture to a boil over medium heat, stirring until the sugar and salt dissolve.
Add the sliced carrots to the saucepan and reduce the heat to low. Simmer for about 5 minutes, just until the carrots are slightly tender but still crisp.
Remove the saucepan from heat and let the mixture cool for about 10 minutes.
Transfer the carrots and pickling liquid to a clean glass jar or container. Make sure the carrots are fully submerged in the liquid.
Seal the jar and refrigerate for at least 24 hours before serving to allow the flavors to develop. The pickled carrots can be stored in the refrigerator for up to 2 weeks.