imo there's a lot that's planned in advance, but in most of those cases, not from beginning to end the way people usually imply on like reddit or whatever, more likely 1-3 arcs ahead of time (ofc, there's always gonna be some stuff that is planned long-term, and some stuff that's more improvised as well)
One thing I think is pretty cool about this is that you can even see Oda's research for the next arc in the ones immediately preceding it sometimes, like some cultural/historic references during Alabasta's publication (especially the 2nd half) that overlap heavily with Skypiea,
e.x. The color spread below with Luffy wearing clothes with symbols from various Native American cultures (some of which are actually just straight up reused in Skypiea), and Nami wearing camo similar to how she does during the angel beach part of the arc. (
Chapter 175, so this was during the Mr. Prince/Rainbase portion of Alabasta)
There's also locations in India referenced in Alabasta's architecture (the clock tower being based on the Ghanta Gar, and Alubarna palace being half-based on the Taj Mahal, both verified in Rurubu One Piece), that might have been related to him researching locations for Jaya in the next arc, an island named after a Sanskrit term* that also features South Asian references in architecture and attire.
* (much like the city of Shandora/Chandra, something far more noticeable when written in katakana)
It all makes sense when you realize how much weekly authors are pressed for time, and how much it'd help to just refer back to the same history and architecture books (or even same sketchbooks/concepts) whenever it makes enough sense. Plus, he seems to like not wasting reusable things in the story to begin with (like how background characters and old locations are constantly kept as a part of the story). Though tbr, it also seems harder to spot now, might be since Oda has a lot more research under his belt after so many arcs, maybe he'd be less inclined to pull from a shallow bag and reuse IRL references from one arc to the immediate next arc, or to 'show his hand' early with his research topics meaningfully