Wipe off the excess pine oil with toilet paper or a paper towel and discard.
Combine 1 tablespoon of chlorine bleach with 1/2 gallon of water. Dip a rag in the mix and wring it out. Wipe down the toilet seat, concentrating on the stained area. You can also use a bathroom spray that contains bleach but avoid abrasive cleaners. Wait 15 minutes before rinsing so the bleach has time to remove the pine oil stain.
Rinse the plastic toilet seat with a wet rag, then dry with a dry rag. Dry the hinges as well.