Are essential oils safe to use?

For the main part, yes. Essential oils are derived from plants, but do be cautious and use it properly. There are amazing therapeutic properties - though it may irritate the skin (for example) for others or even some animals (which have a different system than humans).  It would be best to consult with a physician/professional to find out if it is suitable for you (or an animal) to use in your situation as it may not be for all.