Yes, Oda is a great writer. He's created fascinating characters, settings, and plots using rhythm, FORESHADOWING!!!1, allusions, imagery, wordplay, gags, etc etc. He is an excellent writer. He's been working on the idea that eventually became OP since he was in junior high, and it's obvious how much he loves this universe and the characters he's made.
He's made some on-the-fly decisions that haven't quite worked out, dropped things that seemed important, dragged on things that seemed unimportant, and pissed off his readers plenty of times. I agree with
@Jew D. Boy that some of those decisions might not have been the way he wanted the story to go, but we'll probably never know. From what we see of him in SBS and interviews, he's a bit childish and even flippant sometimes.
You should also consider the unique challenge he has in writing such a long-running serial. It's near impossible to retcon something, and each chapter has a deadline, whether or not the story is ready. Serials are way different than completed works.
But there are better writers out there, and there are more polished works out there. What Oda is is a great
storyteller. He's the kind of person who can come up with stories that fully immerse the audience while they're all gathered around a campfire, you know? The proof of his storytelling greatness is how many people get frustrated with OP but keep coming back because they want to know what happens and because they care so much about his characters.
A great writer with no story to tell is a shit writer. But a great storyteller can get away with bumpy writing if the story is good enough.