Can i use steel cut oats for my oatmeal bath ?

Short answer yes: and long answer is steel cut oats are oats with most of the original parts there, while rolled oats are steamed and rolled over and over again, i.e. more processed. For a bath, you should not care as much, but ingesting steel cut oats generally give you healthier loads of whole-grain fiber and protein as well as lower glycemic index than instant (and highly processed) oatmeal.

Yes -colloidal oats: For an oatmeal bath you can use unflavored instant oatmeal, quick oats, or regular slow cooking oats. Blend them at highest speed into a fine powder. You can use a blender, food processor or coffee grinder. Add ½ to 1 ½ cups to warm (not hot) water & soak for 15 to 20 minutes. Use 1/3rd cup oatmeal for babies.