If there's one thing Oda still does, and better than ever, it's fighting. I think it's a little impaired by his direction for the story, but his ability to put one together is remarkable. I really don't follow all this new school shit, so this isn't to compare. I'm just saying on his own terms he's pretty brilliant.
1. Style
"Styles make fights." It's the most important actual thing in the real world and it always impresses me that a nerdy shut-in like Oda understands so well. One Piece is a world where no two people fight alike: look at CP9, or Kaido/Yamato. The way characters apply abilities that matter. I mentioned CP9 because this is a huge lesson, how a linear power scale failed to accurately compare what each of those villains brought to the table.
Likewise you have matchups. Satori managed to take on Luffy, Sanji, and Usopp while the higher ranked Ohm lost to Zoro alone. Zoro bitch smacks the jitte that cripples Luffy, but then falls to the lightning that Luffy continues to no-sell. It seems like a lot of fans fell for the rock-paper-scissors shit from earlier, but Oda has always been much deeper than that even if he tends to work his way out of a corner with sheer gags. The approach taken to Crocodile was irrelevant to Daz, and relatively weak fighters such as Foxy use cunning and initiative to contend with much stronger combatants. Context and character dynamics make the world go round.
2. Unorthodox nature
Like the rock-papers-scissors approach to weaknesses, the fight pattern is a gift and a curse. Arlong Park and Alabasta are so iconic that people SWEAR there's something wrong when they don't see that 1-2-3-4 matchup shit. "The second command isn't a swordsman, oh that's weird. They don't have a robot, who tf will Franky fight? Where's the idiot/shooter for Usopp?" and so on.
When I first discovered English OP, the-grandline was up to date on the databooks. Looking at the fight catalogue intrigued me because before I'd even seen the 1-2-3-4 matchup in Arlong Park, I noticed an unpredictability that sets the tone. Some storylines had some characters not participate in any fighting. I saw Drum's info as a great example, because by no means was it DBZ where Wapol only fought Luffy after beating the shit outta the others. Could he even beat Sanji, really? I was also intrigued because the moves listed weren't Dragon Punch, Kamehameha, etc. It was a bunch of fluid, weird ass utility moves mixed in with a few staples.
3. Luffy
He'll always be the leading example, obviously. Oda loves him and loves doing his fights, again to a fault because he's like 80% of the fighting time for the main cast. Luffy is his innovation in a nutshell and surprisingly a great fighter if still a poor strategist. Super adaptable and intuitive. A natural brawler with a very fun and all-ages friendly power; Gomu Gomu no Buh is one of my favorite moments ever. Not only has Oda learned beautifully from masters as far back as Segar to peers like Kishimoto, but he also takes something like the signature "meter" move Gatling and turns it on its head with variations in drawing and modifiers.
Again Oda tends to show an eerie understanding of actual fighting. Luffy's Pistol isn't just a signature, it's a legitimate jab: he uses it to poke and set up harder shots. He shows brilliant hand to hand in cases like using his foot to stop his punch on Crocodile, or faking and setting up his flurry against the clever Doflamingo. Sanji hits a beautiful stature smash on Jabura and Zoro typically outsmarts his opponents, but Oda really does save his best for Luffy.
4. Design
The layout is the best part. The storytelling and flow are often pretty incredible. This recent arc showed some really cool shit for a lot of very different characters. I happen to random have volumes 9 and 72 next to me... although he's traded in his clean and unique storytelling style, the grittier, more complete art is necessary for the shift in emphasis.
5. Haki
Oda's made some weird moves with this one, but here we are. I don't like the depiction or lack of explanations, but it all results in some crazy dramatic scenes. Great juggling act between showing that it makes or breaks the new world, and showing that being full of yourself is the way to get slept.