The essential oils not safe for dogs are garlic, clover, oregano, rosemary, thyme, tea tree, and wintergreen oil. As for cats, the essential oils that arent safe to use are eucalyptus, clove, lavender, spruce, cassia, lemon, birch, and more. The common safe oils to diffuse are chamomile, clary sage, cedarwood, marjoram, and myrrh oil.