Many essential oils that we use are very dangerous for cats:

These include:
Peppermint, Lemon Oil, Lavender Oil, Melaleuca Oil, Tea Tree Oil and Cinnamon Bark Oil, pennyroyal, hyssop, thyme, eucalyptus, tea tree, wormwood, camphor, sage, rosemary and tansy.

Even having the oils on our hands then petting the cat can be dangerous, as they will ingest them when they lick their fur.

More about that here:
http://hubpages.com/animals/Cats-and...those-to-avoid