Melt a little bit of chocolate chips in a bowl. You won't need very much, maybe 1/4 cup or less depending on how many spiders you make.
Place the pretzel rods on both sides of the oreos to form the spider legs. You may need to break the rods in half.
Use a toothpick to place a little bit of melted chocolate where the eyes will go to help them stick. Place the eye candies on top immediately. Place in the fridge until the chocolate sets
Notes
If you are making these with small kids, I suggest buying double stuf or mega stuf oreos. It makes it a lot easier for them to stick the pretzels inside without the cookie breaking.