In my world I'm working on, magic is applied mana, as in many others. The trick is that mana is both a byproduct of all life, and it mutates living beings, reducing their intelligence but strengthening/mutating the body if they get too much too quickly, or increasing intelligence if they get just a tiny bit too much. Combined with more complex life being impacted faster, you end up with humans being confined to nearly empty areas otherwise they mutate into mindless beasts, while some can slowly adjust to higher mana density and move to the outskirts, attempting to push back the wilderness. As a being produces mana, it also releases some of its stored mana into the atmosphere, and absorbs some as well, depending on its physical and mental status. Likewise, when a being dies, a large portion of its stored mana is trapped within the body, with the rest released into the atmosphere. This means that if you kill a monster, you have to destroy the body, otherwise other life might consume it and become monsters themselves.
Mages tend to try to expose themselves to just a tiny bit too much mana to improve their intelligence, but this means that though they are powerful with their magic, they are very easily killed. A very small subset of mages instead intentionally expose themselves to far too much magic, sacrificing their own intelligence in order for their bodies to be changed to stronger forms. The vast majority of mages consider these insane madmen, but for some odd reason, they tend to survive far longer anywhere outside of cities. This does however mean they are less potent at casting magic than those who focused purely on increasing their intelligence.
Additionally, skill reduces mana cost, but the same spell cast by a novice or archmage will do the same thing. It is merely that a novice will waste far more mana, which is released instead as 'magefire' around the mage. The more skilled a mage in the components their spell is composed of, the less magefire. So you don't fear a flashy mage as much as one that doesn't show any reaction. The exception to this is magical/mutant creations, who are always perfectly efficient at their innate skills - but mages can learn to replicate and improve on those skills, while the mutant creatures have no way to improve without becoming intelligent first.
These traits combined together also end up recreating the mages' familiar situation -- if a mage has a pet, the higher mana density around the mage results in the pet becoming intelligent over time, though they are also very hard to replace as the mage grows more powerful, as their mana density would mutate or kill most creatures without restricting it. For this reason, many mages intentionally waste mana to keep the local mana density low. Likewise, if a mage knows they will die, they will typically try to spend all their mana so they don't cause more monsters after death. This can be as simple as making a ton of enchanted objects if in a city, or more or less going nova if they were in combat.
Additionally, this system means even someone with no talent for magic can benefit greatly from mana. A number of rangers regularly go through cycles of light/heavy overload to increase their body's strength so they can successfully hunt monsters, for example, and warriors benefit from the same trick as well.
To become OP is rather simple in theory, but quite difficult due to how it messes with your mind: first you expose yourself to a small amount of mana overload to increase your intelligence, then high overload to reduce intelligence and increase body durability. While recovering (assuming you didn't go too far and end up mindless) you cast spells continually while trying to understand as much as possible about the process - this leads to less mana wasted, and thus you can add more power to the spells in the future for the same mana cost. Some mages focus on a single form of magic, such as light rays and bolts, and can pack dozens of times more power for the same mana as a novice, while others spread their focus and may only get double the efficiency of a novice. If you pick something you can easily reuse in many situations, such as pure force projection, body enhancement, or mental magic, you can focus on that, get insanely high efficiency, and then makeshift a ton of different uses of it.