Summarize:

Use the same essential oil that you started with to refresh your potpourri. If you can’t remember what you used or you don’t have it anymore, use an essential oil fragrance that complements what it used to smell like. There will probably be a little bit of essential oil left in your mix, so the scents will combine slightly.  Lemon oil and orange oil are great for citrus potpourri. Rose or lavender oil are both good for floral potpourri.
Add 2 to 3 drops of essential oil to your potpourri.