Orange Potions restore a fixed amount of HP upon use. Their cool-down time is 60 seconds and is shared with Red Potions. They used to be available from Merchant NPCs.
Orange Potions are remnants of a time when Red and Orange Potions behaved differently. Red Potions had a heal-over-time effect like that of the Cleric skill Restoration, while Orange Potions restored HP instantly.
Later on, Red Potions were modified to have an instant healing effect, too and with E5, merchant NPCs eventually stopped selling the now redundant Orange Potions. Items already in-game have not been removed. For example, some Trainee Island quests still award orange potions.