Personally, I'd avoid using essential oils when preggers and/or nursing. I think I can make do without them. But if you do choose to use them, always dilute them with a carrier oil like coconut or hemp. Some essential oils that are generally safe for nursing include chamomile, lavender, tea tree and ginger. But use them sparingly and never ingest them. You can use them topically but always with a carrier oil.
This is my friend's go-to article when she wants to get essential oils: http://www.thehippyhomemaker.com/using-essential-oils-safely-for-pregnant-nursing-mamas/ There is a list of oils to avoid, and a list that are safe to use. She uses these sparingly and very occasionally, and as the article mentioned, always dilute the oil when using.