One Up instantly restores the Hero's HP. Levelling up One Up does nothing.
One Up is of little use. Aside from the great difficulty in navigating the Dark Forest to learn One Up in the first place, the player likely already has Heal at a very high level, and can learn Super Heal in Ebon Keep. The latter in particular is superior to One Up in every way except ingredient consumption.
Acid Rain - Atlas - Barrier - Call Up - Corrosion - Crush - Cure - Defend - Double Drain - Drain - Energize - Escape - Explosion - Fireball - Fire Power - Flash - Hard Ball - Heal - Lance - Laser - Levitate - Lightning Storm - Miracle Cure - Nitro - One Up - Reflect - Regrowth - Revealer - Revive - Slow Burn - Speed - Sting - Stop - Super Heal