The quickest way to make these is by using an "Oreo cookie chocolate candy mold" that can be found in most cake/candy supply stores or online. -->
If you don't have a mold but want that perfectly smooth look, you can use disposable plastic cups!
Spoon about a tablespoon or two of melted chocolate or candy melts (available at most craft stores) into the bottom of the cup. Place your Oreo cookie in the center, pressing down slightly (but not all the way to the bottom) so the chocolate comes up the sides of the cookie, with a layer of chocolate at the bottom (which will become the top). Spoon more melted chocolate on top. Tap the cup on the counter top a few times to release air bubbles. Place cup in the freezer to dry and set, about 10 minutes. Chocolate will darken in color as it dries. You should not be able to see light spots through the plastic, these are spots that have not dried fully yet. Once chocolate has dried and hardened, flip cup upside down with a towel underneath to catch the cookie. Squeeze and bend cup to gently release cookie as you press the bottom of the cup to push the cookie away form the cup. Make sure the chocolate is completely dry and set for a smooth chocolate coating. Do not freeze the cookies for too long as this will cause the chocolate to form water droplets and "perspire" once you bring it to room temperature. Do as many cookies as you want at a time.
Note: You can also dip your Oreos in a bowl of melted chocolate using a fork to lift them out, tapping the fork on the side of the bowl to let excess chocolate drip back into the bowl. Place on waxed paper and refrigerate or freeze for about 10 minutes to set.