In the world of One Piece, the Op-Op Fruit is referred to as the "Ultimate Devil Fruit." In the real world, it's referred to as the "OP-OP Fruit." Both monikers have come about for good reason. The Fruit gives its eater a range of physics-defying powers that's broad even by Devil Fruit standards. With it, one can levitate, teleport and rearrange and reassemble matter, to say nothing of its medical applications. It's no wonder that Trafalgar Law, the Fruit's current user, has become one of the most notorious pirates of his generation.
If Luffy or anyone else were ever to confront Law, his powers would make for a difficult matchup, but there are ways to win. For all of the Op-Op Fruit's powers, it has a number of limitations and conditions the user must abide by in order to use it efficiently. Anyone who makes an enemy of Law will have to understand his Devil Fruit's strengths and weaknesses to stand any chance against him.
ROOM
One of the Op-Op Fruit's primary functions gives its user complete control over the placement of objects within a defined spherical space or "ROOM." Anything within the ROOM, be they objects or people, can be levitated, teleported and moved around freely. Law can even use the Fruit to teleport himself and others to a more desirable position, which can be used for offense, defense or escape.
This power seems to make Law sound nigh-untouchable, but there are weaknesses that can be exploited. The biggest potential shortcoming of Law's ROOM is its range. When used by Law, he can only deploy it over a fixed, limited space. He can extend the range quite far, but this comes at a cost to his stamina; if he's not careful, it can also shorten his lifespan. As long as an opponent can avoid being caught in Law's ROOM, they won't be affected by his power.
Shambles
Law adds to his abilities with his Kioku Sword. He can use it to cut or "Amputate" targets into individual pieces without damaging them. This means that he can divide up a person into their head, heart and other body parts and that person can continue to function as though they were still whole.
The mischievous Surgeon of Death can even attach and reattach divided-up parts as he sees fit. This has led to Marine soldiers whose top halves were connected to each other or severed heads being attached to inanimate objects. He can even rearrange the souls of his targets, effectively swapping their entire bodies. What makes this ability really dangerous is that it's up to the user's discretion when the divided parts are returned to their rightful places. If an opponent falls victim to this Amputation, there's not much they can do about it.
Law's Amputation doesn't have a direct counter, but it can be resisted by proficient Haki users. This is seen during the "Dressrosa" arc where Law is unable to cut through Doflamingo's Armament Haki even in the ROOM. He also admits to being unable to use his powers on Kaido or Big Mom due to their vastly superior Haki. This isn't a weakness that can be exploited by just anyone, but it's a noteworthy one.
Medical Applications
Outside of battle, the Op-Op Fruit can be used to heal people. The user can remove malignities from a "patient's" body, including viruses, diseases and even narcotics. Law used this ability to remove his own Amber Lead Syndrome when he first got the Fruit.
Unfortunately, the Op-Op Fruit is not a miracle cure. As with an actual doctor, the user must have in-depth knowledge and expertise regarding the human body and medical science. If they don't, they become a risk to themselves and whoever they operate on.
Immortality Operation
Perhaps one of the greatest and most coveted powers of the Op-Op Fruit is the Immortality Operation. The details aren't clear, but it's rumored that whoever has this surgery performed on them will remain young forever. This is part of what initially drew Doflamingo to the Fruit; he believed that if he could use its full potential in conjunction with Mariejois' secret national treasure, he could potentially rule the world.
Unfortunately, even if the Immortality Surgery is real, it comes with a major drawback. Whoever performs the surgery does so at the cost of their own life. This means that Law can't just perform the surgery on anybody, nor can he use it on himself.