Though owl themes are pretty common for Halloween parties, an owl is a vital character in fantasy novel series like Harry Potter as well, so creating an environment that exudes magic would be perfect. Illuminate the hall in such a way that it is bright enough, yet carries a mysterious and haunted look. Have cardboard owls on fake branches all over, owl-shaped lanterns, and similar other decorations.
Order a customized owl-shaped cake, get an owl engraved on the cake, or have a sugar candy owl on top of the cake. You can bake owl-shaped cupcakes at home, or get them customized as well. Serve baked cookies and biscuits in the shape of an owl, or use existing ingredients and your creativity to create owl-shaped confectioneries.