Can aroma therapy and essential oils be used past the expiry date?

My mother in law had a bunch of both oils that were in a shoe box packed away with other stuff in a larger box and in storage for the last 6-7 years. They havnt been opened since before then, alot of the expiry dates range from 2001-2006. Can I still use them, even if they have a less effect I'll still use them for now, or they completely no good?