Trim and peel the kohlrabi and the carrots. You can use a vegetable peeler on the carrots, but you'll have better luck with a paring knife for the kohlrabi. Trim the tops and bottoms of the kohlrabi. Set the flat bottoms on a cutting surface; use a sharp paring knife to cut down from top to bottom, removing the thick, tough peel. Set the peeled vegetables aside.
In a large bowl, whisk together the oil, vinegar, mustard, and salt until well blended. Add pepper, if you'd like.
Using the large holes on a standing box grater, grate the kohlrabi and carrots into the large bowl.
Toss everything together until the kohlrabi and carrot are evenly coated with the dressing. Taste and add more salt or pepper, if you'd like.