What they do:
Opossums are actually more beneficial than damaging to their environment. Opossums eat undesirable insects, snails and slugs while leaving most vegetation alone. Because of their long rat-like tail, and large mouth with sharp teeth, opossums are often mistaken for giant scary rats. Realize these marsupials are actually shy and non-confrontational. When approached, an opossum will often hiss and bare his 50 teeth. After that initial show, he’ll use this primary means of defense—playing dead until danger has passed.

What you should do:

• Male opossums spend their lives meandering from place to place to feed on what’s available. For that reason if you see an opossum in your yard simply wait awhile and he’ll likely be on his way. 
• Discourage visits by picking up pet food, and securing pet doors 
• Even though opossums rarely raid gardens, chicken coops or garbage cans—make sure lids on trash cans are secure as well. 
• If one does take up residence in a wood pile or under a porch, his nomadic lifestyle will likely move him out before long—but if you must get rid of him try spraying the area with a mixture of 8 ounces of dish detergent, and 8 ounces of castor oil in a gallon of water. 
• Once everyone’s out seal off the area by using chicken wire buried below ground extending to the deck or patio.