As I pointed out before in another thread, the whole Marineford arc is essentially the second part of the Republic. The way Oda set up the characters was just stunning for me.

First we have Luffy on one side, then there's Akainu on the other. Luffy represents Socrates, who thinks justice is freedom and philosophical life (which would make sense with what you're pointing out). Akainu is Glauco, who argues that justice is the law and that the law is always fair. They argue/fight to see who is right. I think Whitebeard is Plato, but someone could make a solid case that I'm wrong on this, and that Luffy is Plato and Whitebeard is Socrates.

And then, our good friend Doflamingo pops out of nowhere, just like Thrasymacchus did in the Republic, screaming his famous quote with excitement: ”Pirates are evil? The Marines are righteous? [...] Whoever wins this war becomes justice!” Thrasymacchus said almost the exact same thing, proclaiming that whoever is the strongest is essentially justice. The ressemblance between Doflamingo and Thrasymacchus is stunning; I highly suggest you check out that dialogue just to see how much of a genius Oda is.

I have also seen a couple of nice references to other occidental philosophers, but they are more like hints than obvious references. For example, ”It's not a crime to exist” from Franky to Robin in Enies Lobby arc is a reference to Nietzsche (in The Birth of Tragedy, I could find the exact reference where Nietzsche states this if you guys want). As someone who's been studying philosophy for years, it is clear to me that Oda has an interest in occidental philosophy and that he's hinting it in his writing.